--- /dev/null
+=head1 NAME
+X<function>
+
+perlfunc - Perl builtin functions
+
+=head1 DESCRIPTION
+
+The functions in this section can serve as terms in an expression.
+They fall into two major categories: list operators and named unary
+operators. These differ in their precedence relationship with a
+following comma. (See the precedence table in L<perlop>.) List
+operators take more than one argument, while unary operators can never
+take more than one argument. Thus, a comma terminates the argument of
+a unary operator, but merely separates the arguments of a list
+operator. A unary operator generally provides scalar context to its
+argument, while a list operator may provide either scalar or list
+contexts for its arguments. If it does both, scalar arguments
+come first and list argument follow, and there can only ever
+be one such list argument. For instance, splice() has three scalar
+arguments followed by a list, whereas gethostbyname() has four scalar
+arguments.
+
+In the syntax descriptions that follow, list operators that expect a
+list (and provide list context for elements of the list) are shown
+with LIST as an argument. Such a list may consist of any combination
+of scalar arguments or list values; the list values will be included
+in the list as if each individual element were interpolated at that
+point in the list, forming a longer single-dimensional list value.
+Commas should separate literal elements of the LIST.
+
+Any function in the list below may be used either with or without
+parentheses around its arguments. (The syntax descriptions omit the
+parentheses.) If you use parentheses, the simple but occasionally
+surprising rule is this: It I<looks> like a function, therefore it I<is> a
+function, and precedence doesn't matter. Otherwise it's a list
+operator or unary operator, and precedence does matter. Whitespace
+between the function and left parenthesis doesn't count, so sometimes
+you need to be careful:
+
+ print 1+2+4; # Prints 7.
+ print(1+2) + 4; # Prints 3.
+ print (1+2)+4; # Also prints 3!
+ print +(1+2)+4; # Prints 7.
+ print ((1+2)+4); # Prints 7.
+
+If you run Perl with the B<-w> switch it can warn you about this. For
+example, the third line above produces:
+
+ print (...) interpreted as function at - line 1.
+ Useless use of integer addition in void context at - line 1.
+
+A few functions take no arguments at all, and therefore work as neither
+unary nor list operators. These include such functions as C<time>
+and C<endpwent>. For example, C<time+86_400> always means
+C<time() + 86_400>.
+
+For functions that can be used in either a scalar or list context,
+nonabortive failure is generally indicated in scalar context by
+returning the undefined value, and in list context by returning the
+empty list.
+
+Remember the following important rule: There is B<no rule> that relates
+the behavior of an expression in list context to its behavior in scalar
+context, or vice versa. It might do two totally different things.
+Each operator and function decides which sort of value would be most
+appropriate to return in scalar context. Some operators return the
+length of the list that would have been returned in list context. Some
+operators return the first value in the list. Some operators return the
+last value in the list. Some operators return a count of successful
+operations. In general, they do what you want, unless you want
+consistency.
+X<context>
+
+A named array in scalar context is quite different from what would at
+first glance appear to be a list in scalar context. You can't get a list
+like C<(1,2,3)> into being in scalar context, because the compiler knows
+the context at compile time. It would generate the scalar comma operator
+there, not the list construction version of the comma. That means it
+was never a list to start with.
+
+In general, functions in Perl that serve as wrappers for system calls ("syscalls")
+of the same name (like chown(2), fork(2), closedir(2), etc.) return
+true when they succeed and C<undef> otherwise, as is usually mentioned
+in the descriptions below. This is different from the C interfaces,
+which return C<-1> on failure. Exceptions to this rule include C<wait>,
+C<waitpid>, and C<syscall>. System calls also set the special C<$!>
+variable on failure. Other functions do not, except accidentally.
+
+Extension modules can also hook into the Perl parser to define new
+kinds of keyword-headed expression. These may look like functions, but
+may also look completely different. The syntax following the keyword
+is defined entirely by the extension. If you are an implementor, see
+L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
+a module, see the module's documentation for details of the syntax that
+it defines.
+
+=head2 Perl Functions by Category
+X<function>
+
+Here are Perl's functions (including things that look like
+functions, like some keywords and named operators)
+arranged by category. Some functions appear in more
+than one place.
+
+=over 4
+
+=item Functions for SCALARs or strings
+X<scalar> X<string> X<character>
+
+C<chomp>, C<chop>, C<chr>, C<crypt>, C<hex>, C<index>, C<lc>, C<lcfirst>,
+C<length>, C<oct>, C<ord>, C<pack>, C<q//>, C<qq//>, C<reverse>,
+C<rindex>, C<sprintf>, C<substr>, C<tr///>, C<uc>, C<ucfirst>, C<y///>
+
+=item Regular expressions and pattern matching
+X<regular expression> X<regex> X<regexp>
+
+C<m//>, C<pos>, C<quotemeta>, C<s///>, C<split>, C<study>, C<qr//>
+
+=item Numeric functions
+X<numeric> X<number> X<trigonometric> X<trigonometry>
+
+C<abs>, C<atan2>, C<cos>, C<exp>, C<hex>, C<int>, C<log>, C<oct>, C<rand>,
+C<sin>, C<sqrt>, C<srand>
+
+=item Functions for real @ARRAYs
+X<array>
+
+C<each>, C<keys>, C<pop>, C<push>, C<shift>, C<splice>, C<unshift>, C<values>
+
+=item Functions for list data
+X<list>
+
+C<grep>, C<join>, C<map>, C<qw//>, C<reverse>, C<sort>, C<unpack>
+
+=item Functions for real %HASHes
+X<hash>
+
+C<delete>, C<each>, C<exists>, C<keys>, C<values>
+
+=item Input and output functions
+X<I/O> X<input> X<output> X<dbm>
+
+C<binmode>, C<close>, C<closedir>, C<dbmclose>, C<dbmopen>, C<die>, C<eof>,
+C<fileno>, C<flock>, C<format>, C<getc>, C<print>, C<printf>, C<read>,
+C<readdir>, C<rewinddir>, C<say>, C<seek>, C<seekdir>, C<select>, C<syscall>,
+C<sysread>, C<sysseek>, C<syswrite>, C<tell>, C<telldir>, C<truncate>,
+C<warn>, C<write>
+
+=item Functions for fixed-length data or records
+
+C<pack>, C<read>, C<syscall>, C<sysread>, C<syswrite>, C<unpack>, C<vec>
+
+=item Functions for filehandles, files, or directories
+X<file> X<filehandle> X<directory> X<pipe> X<link> X<symlink>
+
+C<-I<X>>, C<chdir>, C<chmod>, C<chown>, C<chroot>, C<fcntl>, C<glob>,
+C<ioctl>, C<link>, C<lstat>, C<mkdir>, C<open>, C<opendir>,
+C<readlink>, C<rename>, C<rmdir>, C<stat>, C<symlink>, C<sysopen>,
+C<umask>, C<unlink>, C<utime>
+
+=item Keywords related to the control flow of your Perl program
+X<control flow>
+
+C<caller>, C<continue>, C<die>, C<do>,
+C<dump>, C<eval>, C<evalbytes> C<exit>,
+C<__FILE__>, C<goto>, C<last>, C<__LINE__>, C<next>, C<__PACKAGE__>,
+C<redo>, C<return>, C<sub>, C<__SUB__>, C<wantarray>
+
+C<__SUB__> is only available with a C<use v5.16> (or higher) declaration or
+with the C<"current_sub"> feature (see L<feature>).
+
+=item Keywords related to the switch feature
+
+C<break>, C<continue>, C<default>, C<given>, C<when>
+
+Except for C<continue>, these are available only if you enable the
+C<"switch"> feature or use the C<CORE::> prefix.
+See L<feature> and L<perlsyn/"Switch statements">.
+Alternately, include a C<use v5.10> or later to the current scope. In Perl
+5.14 and earlier, C<continue> required the C<"switch"> feature, like the
+other keywords.
+
+=item Keywords related to scoping
+
+C<caller>, C<import>, C<local>, C<my>, C<our>, C<package>, C<state>, C<use>
+
+C<state> is available only if the C<"state"> feature
+is enabled or if it is prefixed with C<CORE::>. See
+L<feature>. Alternately, include a C<use v5.10> or later to the current scope.
+
+=item Miscellaneous functions
+
+C<defined>, C<dump>, C<eval>, C<evalbytes>,
+C<formline>, C<local>, C<my>, C<our>,
+C<reset>, C<scalar>, C<state>, C<undef>, C<wantarray>
+
+=item Functions for processes and process groups
+X<process> X<pid> X<process id>
+
+C<alarm>, C<exec>, C<fork>, C<getpgrp>, C<getppid>, C<getpriority>, C<kill>,
+C<pipe>, C<qx//>, C<readpipe>, C<setpgrp>,
+C<setpriority>, C<sleep>, C<system>,
+C<times>, C<wait>, C<waitpid>
+
+=item Keywords related to Perl modules
+X<module>
+
+C<do>, C<import>, C<no>, C<package>, C<require>, C<use>
+
+=item Keywords related to classes and object-orientation
+X<object> X<class> X<package>
+
+C<bless>, C<dbmclose>, C<dbmopen>, C<package>, C<ref>, C<tie>, C<tied>,
+C<untie>, C<use>
+
+=item Low-level socket functions
+X<socket> X<sock>
+
+C<accept>, C<bind>, C<connect>, C<getpeername>, C<getsockname>,
+C<getsockopt>, C<listen>, C<recv>, C<send>, C<setsockopt>, C<shutdown>,
+C<socket>, C<socketpair>
+
+=item System V interprocess communication functions
+X<IPC> X<System V> X<semaphore> X<shared memory> X<memory> X<message>
+
+C<msgctl>, C<msgget>, C<msgrcv>, C<msgsnd>, C<semctl>, C<semget>, C<semop>,
+C<shmctl>, C<shmget>, C<shmread>, C<shmwrite>
+
+=item Fetching user and group info
+X<user> X<group> X<password> X<uid> X<gid> X<passwd> X</etc/passwd>
+
+C<endgrent>, C<endhostent>, C<endnetent>, C<endpwent>, C<getgrent>,
+C<getgrgid>, C<getgrnam>, C<getlogin>, C<getpwent>, C<getpwnam>,
+C<getpwuid>, C<setgrent>, C<setpwent>
+
+=item Fetching network info
+X<network> X<protocol> X<host> X<hostname> X<IP> X<address> X<service>
+
+C<endprotoent>, C<endservent>, C<gethostbyaddr>, C<gethostbyname>,
+C<gethostent>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
+C<getprotobyname>, C<getprotobynumber>, C<getprotoent>,
+C<getservbyname>, C<getservbyport>, C<getservent>, C<sethostent>,
+C<setnetent>, C<setprotoent>, C<setservent>
+
+=item Time-related functions
+X<time> X<date>
+
+C<gmtime>, C<localtime>, C<time>, C<times>
+
+=item Functions new in perl5
+X<perl5>
+
+C<abs>, C<bless>, C<break>, C<chomp>, C<chr>, C<continue>, C<default>,
+C<exists>, C<formline>, C<given>, C<glob>, C<import>, C<lc>, C<lcfirst>,
+C<lock>, C<map>, C<my>, C<no>, C<our>, C<prototype>, C<qr//>, C<qw//>, C<qx//>,
+C<readline>, C<readpipe>, C<ref>, C<sub>*, C<sysopen>, C<tie>, C<tied>, C<uc>,
+C<ucfirst>, C<untie>, C<use>, C<when>
+
+* C<sub> was a keyword in Perl 4, but in Perl 5 it is an
+operator, which can be used in expressions.
+
+=item Functions obsoleted in perl5
+
+C<dbmclose>, C<dbmopen>
+
+=back
+
+=head2 Portability
+X<portability> X<Unix> X<portable>
+
+Perl was born in Unix and can therefore access all common Unix
+system calls. In non-Unix environments, the functionality of some
+Unix system calls may not be available or details of the available
+functionality may differ slightly. The Perl functions affected
+by this are:
+
+C<-X>, C<binmode>, C<chmod>, C<chown>, C<chroot>, C<crypt>,
+C<dbmclose>, C<dbmopen>, C<dump>, C<endgrent>, C<endhostent>,
+C<endnetent>, C<endprotoent>, C<endpwent>, C<endservent>, C<exec>,
+C<fcntl>, C<flock>, C<fork>, C<getgrent>, C<getgrgid>, C<gethostbyname>,
+C<gethostent>, C<getlogin>, C<getnetbyaddr>, C<getnetbyname>, C<getnetent>,
+C<getppid>, C<getpgrp>, C<getpriority>, C<getprotobynumber>,
+C<getprotoent>, C<getpwent>, C<getpwnam>, C<getpwuid>,
+C<getservbyport>, C<getservent>, C<getsockopt>, C<glob>, C<ioctl>,
+C<kill>, C<link>, C<lstat>, C<msgctl>, C<msgget>, C<msgrcv>,
+C<msgsnd>, C<open>, C<pipe>, C<readlink>, C<rename>, C<select>, C<semctl>,
+C<semget>, C<semop>, C<setgrent>, C<sethostent>, C<setnetent>,
+C<setpgrp>, C<setpriority>, C<setprotoent>, C<setpwent>,
+C<setservent>, C<setsockopt>, C<shmctl>, C<shmget>, C<shmread>,
+C<shmwrite>, C<socket>, C<socketpair>,
+C<stat>, C<symlink>, C<syscall>, C<sysopen>, C<system>,
+C<times>, C<truncate>, C<umask>, C<unlink>,
+C<utime>, C<wait>, C<waitpid>
+
+For more information about the portability of these functions, see
+L<perlport> and other available platform-specific documentation.
+
+=head2 Alphabetical Listing of Perl Functions
+
+=over
+
+=item -X FILEHANDLE
+X<-r>X<-w>X<-x>X<-o>X<-R>X<-W>X<-X>X<-O>X<-e>X<-z>X<-s>X<-f>X<-d>X<-l>X<-p>
+X<-S>X<-b>X<-c>X<-t>X<-u>X<-g>X<-k>X<-T>X<-B>X<-M>X<-A>X<-C>
+
+=item -X EXPR
+
+=item -X DIRHANDLE
+
+=item -X
+
+A file test, where X is one of the letters listed below. This unary
+operator takes one argument, either a filename, a filehandle, or a dirhandle,
+and tests the associated file to see if something is true about it. If the
+argument is omitted, tests C<$_>, except for C<-t>, which tests STDIN.
+Unless otherwise documented, it returns C<1> for true and C<''> for false, or
+the undefined value if the file doesn't exist. Despite the funny
+names, precedence is the same as any other named unary operator. The
+operator may be any of:
+
+ -r File is readable by effective uid/gid.
+ -w File is writable by effective uid/gid.
+ -x File is executable by effective uid/gid.
+ -o File is owned by effective uid.
+
+ -R File is readable by real uid/gid.
+ -W File is writable by real uid/gid.
+ -X File is executable by real uid/gid.
+ -O File is owned by real uid.
+
+ -e File exists.
+ -z File has zero size (is empty).
+ -s File has nonzero size (returns size in bytes).
+
+ -f File is a plain file.
+ -d File is a directory.
+ -l File is a symbolic link.
+ -p File is a named pipe (FIFO), or Filehandle is a pipe.
+ -S File is a socket.
+ -b File is a block special file.
+ -c File is a character special file.
+ -t Filehandle is opened to a tty.
+
+ -u File has setuid bit set.
+ -g File has setgid bit set.
+ -k File has sticky bit set.
+
+ -T File is an ASCII text file (heuristic guess).
+ -B File is a "binary" file (opposite of -T).
+
+ -M Script start time minus file modification time, in days.
+ -A Same for access time.
+ -C Same for inode change time (Unix, may differ for other platforms)
+
+Example:
+
+ while (<>) {
+ chomp;
+ next unless -f $_; # ignore specials
+ #...
+ }
+
+Note that C<-s/a/b/> does not do a negated substitution. Saying
+C<-exp($foo)> still works as expected, however: only single letters
+following a minus are interpreted as file tests.
+
+These operators are exempt from the "looks like a function rule" described
+above. That is, an opening parenthesis after the operator does not affect
+how much of the following code constitutes the argument. Put the opening
+parentheses before the operator to separate it from code that follows (this
+applies only to operators with higher precedence than unary operators, of
+course):
+
+ -s($file) + 1024 # probably wrong; same as -s($file + 1024)
+ (-s $file) + 1024 # correct
+
+The interpretation of the file permission operators C<-r>, C<-R>,
+C<-w>, C<-W>, C<-x>, and C<-X> is by default based solely on the mode
+of the file and the uids and gids of the user. There may be other
+reasons you can't actually read, write, or execute the file: for
+example network filesystem access controls, ACLs (access control lists),
+read-only filesystems, and unrecognized executable formats. Note
+that the use of these six specific operators to verify if some operation
+is possible is usually a mistake, because it may be open to race
+conditions.
+
+Also note that, for the superuser on the local filesystems, the C<-r>,
+C<-R>, C<-w>, and C<-W> tests always return 1, and C<-x> and C<-X> return 1
+if any execute bit is set in the mode. Scripts run by the superuser
+may thus need to do a stat() to determine the actual mode of the file,
+or temporarily set their effective uid to something else.
+
+If you are using ACLs, there is a pragma called C<filetest> that may
+produce more accurate results than the bare stat() mode bits.
+When under C<use filetest 'access'> the above-mentioned filetests
+test whether the permission can(not) be granted using the
+access(2) family of system calls. Also note that the C<-x> and C<-X> may
+under this pragma return true even if there are no execute permission
+bits set (nor any extra execute permission ACLs). This strangeness is
+due to the underlying system calls' definitions. Note also that, due to
+the implementation of C<use filetest 'access'>, the C<_> special
+filehandle won't cache the results of the file tests when this pragma is
+in effect. Read the documentation for the C<filetest> pragma for more
+information.
+
+The C<-T> and C<-B> switches work as follows. The first block or so of the
+file is examined for odd characters such as strange control codes or
+characters with the high bit set. If too many strange characters (>30%)
+are found, it's a C<-B> file; otherwise it's a C<-T> file. Also, any file
+containing a zero byte in the first block is considered a binary file. If C<-T>
+or C<-B> is used on a filehandle, the current IO buffer is examined
+rather than the first block. Both C<-T> and C<-B> return true on an empty
+file, or a file at EOF when testing a filehandle. Because you have to
+read a file to do the C<-T> test, on most occasions you want to use a C<-f>
+against the file first, as in C<next unless -f $file && -T $file>.
+
+If any of the file tests (or either the C<stat> or C<lstat> operator) is given
+the special filehandle consisting of a solitary underline, then the stat
+structure of the previous file test (or stat operator) is used, saving
+a system call. (This doesn't work with C<-t>, and you need to remember
+that lstat() and C<-l> leave values in the stat structure for the
+symbolic link, not the real file.) (Also, if the stat buffer was filled by
+an C<lstat> call, C<-T> and C<-B> will reset it with the results of C<stat _>).
+Example:
+
+ print "Can do.\n" if -r $a || -w _ || -x _;
+
+ stat($filename);
+ print "Readable\n" if -r _;
+ print "Writable\n" if -w _;
+ print "Executable\n" if -x _;
+ print "Setuid\n" if -u _;
+ print "Setgid\n" if -g _;
+ print "Sticky\n" if -k _;
+ print "Text\n" if -T _;
+ print "Binary\n" if -B _;
+
+As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file
+test operators, in a way that C<-f -w -x $file> is equivalent to
+C<-x $file && -w _ && -f _>. (This is only fancy fancy: if you use
+the return value of C<-f $file> as an argument to another filetest
+operator, no special magic will happen.)
+
+Portability issues: L<perlport/-X>.
+
+=item abs VALUE
+X<abs> X<absolute>
+
+=item abs
+
+Returns the absolute value of its argument.
+If VALUE is omitted, uses C<$_>.
+
+=item accept NEWSOCKET,GENERICSOCKET
+X<accept>
+
+Accepts an incoming socket connect, just as accept(2)
+does. Returns the packed address if it succeeded, false otherwise.
+See the example in L<perlipc/"Sockets: Client/Server Communication">.
+
+On systems that support a close-on-exec flag on files, the flag will
+be set for the newly opened file descriptor, as determined by the
+value of $^F. See L<perlvar/$^F>.
+
+=item alarm SECONDS
+X<alarm>
+X<SIGALRM>
+X<timer>
+
+=item alarm
+
+Arranges to have a SIGALRM delivered to this process after the
+specified number of wallclock seconds has elapsed. If SECONDS is not
+specified, the value stored in C<$_> is used. (On some machines,
+unfortunately, the elapsed time may be up to one second less or more
+than you specified because of how seconds are counted, and process
+scheduling may delay the delivery of the signal even further.)
+
+Only one timer may be counting at once. Each call disables the
+previous timer, and an argument of C<0> may be supplied to cancel the
+previous timer without starting a new one. The returned value is the
+amount of time remaining on the previous timer.
+
+For delays of finer granularity than one second, the Time::HiRes module
+(from CPAN, and starting from Perl 5.8 part of the standard
+distribution) provides ualarm(). You may also use Perl's four-argument
+version of select() leaving the first three arguments undefined, or you
+might be able to use the C<syscall> interface to access setitimer(2) if
+your system supports it. See L<perlfaq8> for details.
+
+It is usually a mistake to intermix C<alarm> and C<sleep> calls, because
+C<sleep> may be internally implemented on your system with C<alarm>.
+
+If you want to use C<alarm> to time out a system call you need to use an
+C<eval>/C<die> pair. You can't rely on the alarm causing the system call to
+fail with C<$!> set to C<EINTR> because Perl sets up signal handlers to
+restart system calls on some systems. Using C<eval>/C<die> always works,
+modulo the caveats given in L<perlipc/"Signals">.
+
+ eval {
+ local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
+ alarm $timeout;
+ $nread = sysread SOCKET, $buffer, $size;
+ alarm 0;
+ };
+ if ($@) {
+ die unless $@ eq "alarm\n"; # propagate unexpected errors
+ # timed out
+ }
+ else {
+ # didn't
+ }
+
+For more information see L<perlipc>.
+
+Portability issues: L<perlport/alarm>.
+
+=item atan2 Y,X
+X<atan2> X<arctangent> X<tan> X<tangent>
+
+Returns the arctangent of Y/X in the range -PI to PI.
+
+For the tangent operation, you may use the C<Math::Trig::tan>
+function, or use the familiar relation:
+
+ sub tan { sin($_[0]) / cos($_[0]) }
+
+The return value for C<atan2(0,0)> is implementation-defined; consult
+your atan2(3) manpage for more information.
+
+Portability issues: L<perlport/atan2>.
+
+=item bind SOCKET,NAME
+X<bind>
+
+Binds a network address to a socket, just as bind(2)
+does. Returns true if it succeeded, false otherwise. NAME should be a
+packed address of the appropriate type for the socket. See the examples in
+L<perlipc/"Sockets: Client/Server Communication">.
+
+=item binmode FILEHANDLE, LAYER
+X<binmode> X<binary> X<text> X<DOS> X<Windows>
+
+=item binmode FILEHANDLE
+
+Arranges for FILEHANDLE to be read or written in "binary" or "text"
+mode on systems where the run-time libraries distinguish between
+binary and text files. If FILEHANDLE is an expression, the value is
+taken as the name of the filehandle. Returns true on success,
+otherwise it returns C<undef> and sets C<$!> (errno).
+
+On some systems (in general, DOS- and Windows-based systems) binmode()
+is necessary when you're not working with a text file. For the sake
+of portability it is a good idea always to use it when appropriate,
+and never to use it when it isn't appropriate. Also, people can
+set their I/O to be by default UTF8-encoded Unicode, not bytes.
+
+In other words: regardless of platform, use binmode() on binary data,
+like images, for example.
+
+If LAYER is present it is a single string, but may contain multiple
+directives. The directives alter the behaviour of the filehandle.
+When LAYER is present, using binmode on a text file makes sense.
+
+If LAYER is omitted or specified as C<:raw> the filehandle is made
+suitable for passing binary data. This includes turning off possible CRLF
+translation and marking it as bytes (as opposed to Unicode characters).
+Note that, despite what may be implied in I<"Programming Perl"> (the
+Camel, 3rd edition) or elsewhere, C<:raw> is I<not> simply the inverse of C<:crlf>.
+Other layers that would affect the binary nature of the stream are
+I<also> disabled. See L<PerlIO>, L<perlrun>, and the discussion about the
+PERLIO environment variable.
+
+The C<:bytes>, C<:crlf>, C<:utf8>, and any other directives of the
+form C<:...>, are called I/O I<layers>. The C<open> pragma can be used to
+establish default I/O layers. See L<open>.
+
+I<The LAYER parameter of the binmode() function is described as "DISCIPLINE"
+in "Programming Perl, 3rd Edition". However, since the publishing of this
+book, by many known as "Camel III", the consensus of the naming of this
+functionality has moved from "discipline" to "layer". All documentation
+of this version of Perl therefore refers to "layers" rather than to
+"disciplines". Now back to the regularly scheduled documentation...>
+
+To mark FILEHANDLE as UTF-8, use C<:utf8> or C<:encoding(UTF-8)>.
+C<:utf8> just marks the data as UTF-8 without further checking,
+while C<:encoding(UTF-8)> checks the data for actually being valid
+UTF-8. More details can be found in L<PerlIO::encoding>.
+
+In general, binmode() should be called after open() but before any I/O
+is done on the filehandle. Calling binmode() normally flushes any
+pending buffered output data (and perhaps pending input data) on the
+handle. An exception to this is the C<:encoding> layer that
+changes the default character encoding of the handle; see L</open>.
+The C<:encoding> layer sometimes needs to be called in
+mid-stream, and it doesn't flush the stream. The C<:encoding>
+also implicitly pushes on top of itself the C<:utf8> layer because
+internally Perl operates on UTF8-encoded Unicode characters.
+
+The operating system, device drivers, C libraries, and Perl run-time
+system all conspire to let the programmer treat a single
+character (C<\n>) as the line terminator, irrespective of external
+representation. On many operating systems, the native text file
+representation matches the internal representation, but on some
+platforms the external representation of C<\n> is made up of more than
+one character.
+
+All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use
+a single character to end each line in the external representation of text
+(even though that single character is CARRIAGE RETURN on old, pre-Darwin
+flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other
+systems like OS/2, DOS, and the various flavors of MS-Windows, your program
+sees a C<\n> as a simple C<\cJ>, but what's stored in text files are the
+two characters C<\cM\cJ>. That means that if you don't use binmode() on
+these systems, C<\cM\cJ> sequences on disk will be converted to C<\n> on
+input, and any C<\n> in your program will be converted back to C<\cM\cJ> on
+output. This is what you want for text files, but it can be disastrous for
+binary files.
+
+Another consequence of using binmode() (on some systems) is that
+special end-of-file markers will be seen as part of the data stream.
+For systems from the Microsoft family this means that, if your binary
+data contain C<\cZ>, the I/O subsystem will regard it as the end of
+the file, unless you use binmode().
+
+binmode() is important not only for readline() and print() operations,
+but also when using read(), seek(), sysread(), syswrite() and tell()
+(see L<perlport> for more details). See the C<$/> and C<$\> variables
+in L<perlvar> for how to manually set your input and output
+line-termination sequences.
+
+Portability issues: L<perlport/binmode>.
+
+=item bless REF,CLASSNAME
+X<bless>
+
+=item bless REF
+
+This function tells the thingy referenced by REF that it is now an object
+in the CLASSNAME package. If CLASSNAME is omitted, the current package
+is used. Because a C<bless> is often the last thing in a constructor,
+it returns the reference for convenience. Always use the two-argument
+version if a derived class might inherit the function doing the blessing.
+SeeL<perlobj> for more about the blessing (and blessings) of objects.
+
+Consider always blessing objects in CLASSNAMEs that are mixed case.
+Namespaces with all lowercase names are considered reserved for
+Perl pragmata. Builtin types have all uppercase names. To prevent
+confusion, you may wish to avoid such package names as well. Make sure
+that CLASSNAME is a true value.
+
+See L<perlmod/"Perl Modules">.
+
+=item break
+
+Break out of a C<given()> block.
+
+This keyword is enabled by the C<"switch"> feature: see
+L<feature> for more information. You can also access it by
+prefixing it with C<CORE::>. Alternately, include a C<use
+v5.10> or later to the current scope.
+
+=item caller EXPR
+X<caller> X<call stack> X<stack> X<stack trace>
+
+=item caller
+
+Returns the context of the current subroutine call. In scalar context,
+returns the caller's package name if there I<is> a caller (that is, if
+we're in a subroutine or C<eval> or C<require>) and the undefined value
+otherwise. In list context, returns
+
+ # 0 1 2
+ ($package, $filename, $line) = caller;
+
+With EXPR, it returns some extra information that the debugger uses to
+print a stack trace. The value of EXPR indicates how many call frames
+to go back before the current one.
+
+ # 0 1 2 3 4
+ ($package, $filename, $line, $subroutine, $hasargs,
+
+ # 5 6 7 8 9 10
+ $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
+ = caller($i);
+
+Here $subroutine may be C<(eval)> if the frame is not a subroutine
+call, but an C<eval>. In such a case additional elements $evaltext and
+C<$is_require> are set: C<$is_require> is true if the frame is created by a
+C<require> or C<use> statement, $evaltext contains the text of the
+C<eval EXPR> statement. In particular, for an C<eval BLOCK> statement,
+$subroutine is C<(eval)>, but $evaltext is undefined. (Note also that
+each C<use> statement creates a C<require> frame inside an C<eval EXPR>
+frame.) $subroutine may also be C<(unknown)> if this particular
+subroutine happens to have been deleted from the symbol table.
+C<$hasargs> is true if a new instance of C<@_> was set up for the frame.
+C<$hints> and C<$bitmask> contain pragmatic hints that the caller was
+compiled with. The C<$hints> and C<$bitmask> values are subject to change
+between versions of Perl, and are not meant for external use.
+
+C<$hinthash> is a reference to a hash containing the value of C<%^H> when the
+caller was compiled, or C<undef> if C<%^H> was empty. Do not modify the values
+of this hash, as they are the actual values stored in the optree.
+
+Furthermore, when called from within the DB package, caller returns more
+detailed information: it sets the list variable C<@DB::args> to be the
+arguments with which the subroutine was invoked.
+
+Be aware that the optimizer might have optimized call frames away before
+C<caller> had a chance to get the information. That means that C<caller(N)>
+might not return information about the call frame you expect it to, for
+C<< N > 1 >>. In particular, C<@DB::args> might have information from the
+previous time C<caller> was called.
+
+Be aware that setting C<@DB::args> is I<best effort>, intended for
+debugging or generating backtraces, and should not be relied upon. In
+particular, as C<@_> contains aliases to the caller's arguments, Perl does
+not take a copy of C<@_>, so C<@DB::args> will contain modifications the
+subroutine makes to C<@_> or its contents, not the original values at call
+time. C<@DB::args>, like C<@_>, does not hold explicit references to its
+elements, so under certain cases its elements may have become freed and
+reallocated for other variables or temporary values. Finally, a side effect
+of the current implementation is that the effects of C<shift @_> can
+I<normally> be undone (but not C<pop @_> or other splicing, I<and> not if a
+reference to C<@_> has been taken, I<and> subject to the caveat about reallocated
+elements), so C<@DB::args> is actually a hybrid of the current state and
+initial state of C<@_>. Buyer beware.
+
+=item chdir EXPR
+X<chdir>
+X<cd>
+X<directory, change>
+
+=item chdir FILEHANDLE
+
+=item chdir DIRHANDLE
+
+=item chdir
+
+Changes the working directory to EXPR, if possible. If EXPR is omitted,
+changes to the directory specified by C<$ENV{HOME}>, if set; if not,
+changes to the directory specified by C<$ENV{LOGDIR}>. (Under VMS, the
+variable C<$ENV{SYS$LOGIN}> is also checked, and used if it is set.) If
+neither is set, C<chdir> does nothing. It returns true on success,
+false otherwise. See the example under C<die>.
+
+On systems that support fchdir(2), you may pass a filehandle or
+directory handle as the argument. On systems that don't support fchdir(2),
+passing handles raises an exception.
+
+=item chmod LIST
+X<chmod> X<permission> X<mode>
+
+Changes the permissions of a list of files. The first element of the
+list must be the numeric mode, which should probably be an octal
+number, and which definitely should I<not> be a string of octal digits:
+C<0644> is okay, but C<"0644"> is not. Returns the number of files
+successfully changed. See also L</oct> if all you have is a string.
+
+ $cnt = chmod 0755, "foo", "bar";
+ chmod 0755, @executables;
+ $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
+ # --w----r-T
+ $mode = "0644"; chmod oct($mode), "foo"; # this is better
+ $mode = 0644; chmod $mode, "foo"; # this is best
+
+On systems that support fchmod(2), you may pass filehandles among the
+files. On systems that don't support fchmod(2), passing filehandles raises
+an exception. Filehandles must be passed as globs or glob references to be
+recognized; barewords are considered filenames.
+
+ open(my $fh, "<", "foo");
+ my $perm = (stat $fh)[2] & 07777;
+ chmod($perm | 0600, $fh);
+
+You can also import the symbolic C<S_I*> constants from the C<Fcntl>
+module:
+
+ use Fcntl qw( :mode );
+ chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
+ # Identical to the chmod 0755 of the example above.
+
+Portability issues: L<perlport/chmod>.
+
+=item chomp VARIABLE
+X<chomp> X<INPUT_RECORD_SEPARATOR> X<$/> X<newline> X<eol>
+
+=item chomp( LIST )
+
+=item chomp
+
+This safer version of L</chop> removes any trailing string
+that corresponds to the current value of C<$/> (also known as
+$INPUT_RECORD_SEPARATOR in the C<English> module). It returns the total
+number of characters removed from all its arguments. It's often used to
+remove the newline from the end of an input record when you're worried
+that the final record may be missing its newline. When in paragraph
+mode (C<$/ = "">), it removes all trailing newlines from the string.
+When in slurp mode (C<$/ = undef>) or fixed-length record mode (C<$/> is
+a reference to an integer or the like; see L<perlvar>) chomp() won't
+remove anything.
+If VARIABLE is omitted, it chomps C<$_>. Example:
+
+ while (<>) {
+ chomp; # avoid \n on last field
+ @array = split(/:/);
+ # ...
+ }
+
+If VARIABLE is a hash, it chomps the hash's values, but not its keys.
+
+You can actually chomp anything that's an lvalue, including an assignment:
+
+ chomp($cwd = `pwd`);
+ chomp($answer = <STDIN>);
+
+If you chomp a list, each element is chomped, and the total number of
+characters removed is returned.
+
+Note that parentheses are necessary when you're chomping anything
+that is not a simple variable. This is because C<chomp $cwd = `pwd`;>
+is interpreted as C<(chomp $cwd) = `pwd`;>, rather than as
+C<chomp( $cwd = `pwd` )> which you might expect. Similarly,
+C<chomp $a, $b> is interpreted as C<chomp($a), $b> rather than
+as C<chomp($a, $b)>.
+
+=item chop VARIABLE
+X<chop>
+
+=item chop( LIST )
+
+=item chop
+
+Chops off the last character of a string and returns the character
+chopped. It is much more efficient than C<s/.$//s> because it neither
+scans nor copies the string. If VARIABLE is omitted, chops C<$_>.
+If VARIABLE is a hash, it chops the hash's values, but not its keys.
+
+You can actually chop anything that's an lvalue, including an assignment.
+
+If you chop a list, each element is chopped. Only the value of the
+last C<chop> is returned.
+
+Note that C<chop> returns the last character. To return all but the last
+character, use C<substr($string, 0, -1)>.
+
+See also L</chomp>.
+
+=item chown LIST
+X<chown> X<owner> X<user> X<group>
+
+Changes the owner (and group) of a list of files. The first two
+elements of the list must be the I<numeric> uid and gid, in that
+order. A value of -1 in either position is interpreted by most
+systems to leave that value unchanged. Returns the number of files
+successfully changed.
+
+ $cnt = chown $uid, $gid, 'foo', 'bar';
+ chown $uid, $gid, @filenames;
+
+On systems that support fchown(2), you may pass filehandles among the
+files. On systems that don't support fchown(2), passing filehandles raises
+an exception. Filehandles must be passed as globs or glob references to be
+recognized; barewords are considered filenames.
+
+Here's an example that looks up nonnumeric uids in the passwd file:
+
+ print "User: ";
+ chomp($user = <STDIN>);
+ print "Files: ";
+ chomp($pattern = <STDIN>);
+
+ ($login,$pass,$uid,$gid) = getpwnam($user)
+ or die "$user not in passwd file";
+
+ @ary = glob($pattern); # expand filenames
+ chown $uid, $gid, @ary;
+
+On most systems, you are not allowed to change the ownership of the
+file unless you're the superuser, although you should be able to change
+the group to any of your secondary groups. On insecure systems, these
+restrictions may be relaxed, but this is not a portable assumption.
+On POSIX systems, you can detect this condition this way:
+
+ use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
+ $can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
+
+Portability issues: L<perlport/chmod>.
+
+=item chr NUMBER
+X<chr> X<character> X<ASCII> X<Unicode>
+
+=item chr
+
+Returns the character represented by that NUMBER in the character set.
+For example, C<chr(65)> is C<"A"> in either ASCII or Unicode, and
+chr(0x263a) is a Unicode smiley face.
+
+Negative values give the Unicode replacement character (chr(0xfffd)),
+except under the L<bytes> pragma, where the low eight bits of the value
+(truncated to an integer) are used.
+
+If NUMBER is omitted, uses C<$_>.
+
+For the reverse, use L</ord>.
+
+Note that characters from 128 to 255 (inclusive) are by default
+internally not encoded as UTF-8 for backward compatibility reasons.
+
+See L<perlunicode> for more about Unicode.
+
+=item chroot FILENAME
+X<chroot> X<root>
+
+=item chroot
+
+This function works like the system call by the same name: it makes the
+named directory the new root directory for all further pathnames that
+begin with a C</> by your process and all its children. (It doesn't
+change your current working directory, which is unaffected.) For security
+reasons, this call is restricted to the superuser. If FILENAME is
+omitted, does a C<chroot> to C<$_>.
+
+Portability issues: L<perlport/chroot>.
+
+=item close FILEHANDLE
+X<close>
+
+=item close
+
+Closes the file or pipe associated with the filehandle, flushes the IO
+buffers, and closes the system file descriptor. Returns true if those
+operations succeed and if no error was reported by any PerlIO
+layer. Closes the currently selected filehandle if the argument is
+omitted.
+
+You don't have to close FILEHANDLE if you are immediately going to do
+another C<open> on it, because C<open> closes it for you. (See
+L<open|/open FILEHANDLE>.) However, an explicit C<close> on an input file resets the line
+counter (C<$.>), while the implicit close done by C<open> does not.
+
+If the filehandle came from a piped open, C<close> returns false if one of
+the other syscalls involved fails or if its program exits with non-zero
+status. If the only problem was that the program exited non-zero, C<$!>
+will be set to C<0>. Closing a pipe also waits for the process executing
+on the pipe to exit--in case you wish to look at the output of the pipe
+afterwards--and implicitly puts the exit status value of that command into
+C<$?> and C<${^CHILD_ERROR_NATIVE}>.
+
+If there are multiple threads running, C<close> on a filehandle from a
+piped open returns true without waiting for the child process to terminate,
+if the filehandle is still open in another thread.
+
+Closing the read end of a pipe before the process writing to it at the
+other end is done writing results in the writer receiving a SIGPIPE. If
+the other end can't handle that, be sure to read all the data before
+closing the pipe.
+
+Example:
+
+ open(OUTPUT, '|sort >foo') # pipe to sort
+ or die "Can't start sort: $!";
+ #... # print stuff to output
+ close OUTPUT # wait for sort to finish
+ or warn $! ? "Error closing sort pipe: $!"
+ : "Exit status $? from sort";
+ open(INPUT, 'foo') # get sort's results
+ or die "Can't open 'foo' for input: $!";
+
+FILEHANDLE may be an expression whose value can be used as an indirect
+filehandle, usually the real filehandle name or an autovivified handle.
+
+=item closedir DIRHANDLE
+X<closedir>
+
+Closes a directory opened by C<opendir> and returns the success of that
+system call.
+
+=item connect SOCKET,NAME
+X<connect>
+
+Attempts to connect to a remote socket, just like connect(2).
+Returns true if it succeeded, false otherwise. NAME should be a
+packed address of the appropriate type for the socket. See the examples in
+L<perlipc/"Sockets: Client/Server Communication">.
+
+=item continue BLOCK
+X<continue>
+
+=item continue
+
+When followed by a BLOCK, C<continue> is actually a
+flow control statement rather than a function. If
+there is a C<continue> BLOCK attached to a BLOCK (typically in a C<while> or
+C<foreach>), it is always executed just before the conditional is about to
+be evaluated again, just like the third part of a C<for> loop in C. Thus
+it can be used to increment a loop variable, even when the loop has been
+continued via the C<next> statement (which is similar to the C C<continue>
+statement).
+
+C<last>, C<next>, or C<redo> may appear within a C<continue>
+block; C<last> and C<redo> behave as if they had been executed within
+the main block. So will C<next>, but since it will execute a C<continue>
+block, it may be more entertaining.
+
+ while (EXPR) {
+ ### redo always comes here
+ do_something;
+ } continue {
+ ### next always comes here
+ do_something_else;
+ # then back the top to re-check EXPR
+ }
+ ### last always comes here
+
+Omitting the C<continue> section is equivalent to using an
+empty one, logically enough, so C<next> goes directly back
+to check the condition at the top of the loop.
+
+When there is no BLOCK, C<continue> is a function that
+falls through the current C<when> or C<default> block instead of iterating
+a dynamically enclosing C<foreach> or exiting a lexically enclosing C<given>.
+In Perl 5.14 and earlier, this form of C<continue> was
+only available when the C<"switch"> feature was enabled.
+See L<feature> and L<perlsyn/"Switch statements"> for more
+information.
+
+=item cos EXPR
+X<cos> X<cosine> X<acos> X<arccosine>
+
+=item cos
+
+Returns the cosine of EXPR (expressed in radians). If EXPR is omitted,
+takes the cosine of C<$_>.
+
+For the inverse cosine operation, you may use the C<Math::Trig::acos()>
+function, or use this relation:
+
+ sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
+
+=item crypt PLAINTEXT,SALT
+X<crypt> X<digest> X<hash> X<salt> X<plaintext> X<password>
+X<decrypt> X<cryptography> X<passwd> X<encrypt>
+
+Creates a digest string exactly like the crypt(3) function in the C
+library (assuming that you actually have a version there that has not
+been extirpated as a potential munition).
+
+crypt() is a one-way hash function. The PLAINTEXT and SALT are turned
+into a short string, called a digest, which is returned. The same
+PLAINTEXT and SALT will always return the same string, but there is no
+(known) way to get the original PLAINTEXT from the hash. Small
+changes in the PLAINTEXT or SALT will result in large changes in the
+digest.
+
+There is no decrypt function. This function isn't all that useful for
+cryptography (for that, look for F<Crypt> modules on your nearby CPAN
+mirror) and the name "crypt" is a bit of a misnomer. Instead it is
+primarily used to check if two pieces of text are the same without
+having to transmit or store the text itself. An example is checking
+if a correct password is given. The digest of the password is stored,
+not the password itself. The user types in a password that is
+crypt()'d with the same salt as the stored digest. If the two digests
+match, the password is correct.
+
+When verifying an existing digest string you should use the digest as
+the salt (like C<crypt($plain, $digest) eq $digest>). The SALT used
+to create the digest is visible as part of the digest. This ensures
+crypt() will hash the new string with the same salt as the digest.
+This allows your code to work with the standard L<crypt|/crypt> and
+with more exotic implementations. In other words, assume
+nothing about the returned string itself nor about how many bytes
+of SALT may matter.
+
+Traditionally the result is a string of 13 bytes: two first bytes of
+the salt, followed by 11 bytes from the set C<[./0-9A-Za-z]>, and only
+the first eight bytes of PLAINTEXT mattered. But alternative
+hashing schemes (like MD5), higher level security schemes (like C2),
+and implementations on non-Unix platforms may produce different
+strings.
+
+When choosing a new salt create a random two character string whose
+characters come from the set C<[./0-9A-Za-z]> (like C<join '', ('.',
+'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]>). This set of
+characters is just a recommendation; the characters allowed in
+the salt depend solely on your system's crypt library, and Perl can't
+restrict what salts C<crypt()> accepts.
+
+Here's an example that makes sure that whoever runs this program knows
+their password:
+
+ $pwd = (getpwuid($<))[1];
+
+ system "stty -echo";
+ print "Password: ";
+ chomp($word = <STDIN>);
+ print "\n";
+ system "stty echo";
+
+ if (crypt($word, $pwd) ne $pwd) {
+ die "Sorry...\n";
+ } else {
+ print "ok\n";
+ }
+
+Of course, typing in your own password to whoever asks you
+for it is unwise.
+
+The L<crypt|/crypt> function is unsuitable for hashing large quantities
+of data, not least of all because you can't get the information
+back. Look at the L<Digest> module for more robust algorithms.
+
+If using crypt() on a Unicode string (which I<potentially> has
+characters with codepoints above 255), Perl tries to make sense
+of the situation by trying to downgrade (a copy of)
+the string back to an eight-bit byte string before calling crypt()
+(on that copy). If that works, good. If not, crypt() dies with
+C<Wide character in crypt>.
+
+Portability issues: L<perlport/crypt>.
+
+=item dbmclose HASH
+X<dbmclose>
+
+[This function has been largely superseded by the C<untie> function.]
+
+Breaks the binding between a DBM file and a hash.
+
+Portability issues: L<perlport/dbmclose>.
+
+=item dbmopen HASH,DBNAME,MASK
+X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>
+
+[This function has been largely superseded by the
+L<tie|/tie VARIABLE,CLASSNAME,LIST> function.]
+
+This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a
+hash. HASH is the name of the hash. (Unlike normal C<open>, the first
+argument is I<not> a filehandle, even though it looks like one). DBNAME
+is the name of the database (without the F<.dir> or F<.pag> extension if
+any). If the database does not exist, it is created with protection
+specified by MASK (as modified by the C<umask>). If your system supports
+only the older DBM functions, you may make only one C<dbmopen> call in your
+program. In older versions of Perl, if your system had neither DBM nor
+ndbm, calling C<dbmopen> produced a fatal error; it now falls back to
+sdbm(3).
+
+If you don't have write access to the DBM file, you can only read hash
+variables, not set them. If you want to test whether you can write,
+either use file tests or try setting a dummy hash entry inside an C<eval>
+to trap the error.
+
+Note that functions such as C<keys> and C<values> may return huge lists
+when used on large DBM files. You may prefer to use the C<each>
+function to iterate over large DBM files. Example:
+
+ # print out history file offsets
+ dbmopen(%HIST,'/usr/lib/news/history',0666);
+ while (($key,$val) = each %HIST) {
+ print $key, ' = ', unpack('L',$val), "\n";
+ }
+ dbmclose(%HIST);
+
+See also L<AnyDBM_File> for a more general description of the pros and
+cons of the various dbm approaches, as well as L<DB_File> for a particularly
+rich implementation.
+
+You can control which DBM library you use by loading that library
+before you call dbmopen():
+
+ use DB_File;
+ dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
+ or die "Can't open netscape history file: $!";
+
+Portability issues: L<perlport/dbmopen>.
+
+=item default BLOCK
+
+Within a C<foreach> or a C<given>, a C<default> BLOCK acts like a C<when>
+that's always true. Only available after Perl 5.10, and only if the
+C<switch> feature has been requested or if the keyword is prefixed with
+C<CORE::>. See L</when>.
+
+=item defined EXPR
+X<defined> X<undef> X<undefined>
+
+=item defined
+
+Returns a Boolean value telling whether EXPR has a value other than
+the undefined value C<undef>. If EXPR is not present, C<$_> is
+checked.
+
+Many operations return C<undef> to indicate failure, end of file,
+system error, uninitialized variable, and other exceptional
+conditions. This function allows you to distinguish C<undef> from
+other values. (A simple Boolean test will not distinguish among
+C<undef>, zero, the empty string, and C<"0">, which are all equally
+false.) Note that since C<undef> is a valid scalar, its presence
+doesn't I<necessarily> indicate an exceptional condition: C<pop>
+returns C<undef> when its argument is an empty array, I<or> when the
+element to return happens to be C<undef>.
+
+You may also use C<defined(&func)> to check whether subroutine C<&func>
+has ever been defined. The return value is unaffected by any forward
+declarations of C<&func>. A subroutine that is not defined
+may still be callable: its package may have an C<AUTOLOAD> method that
+makes it spring into existence the first time that it is called; see
+L<perlsub>.
+
+Use of C<defined> on aggregates (hashes and arrays) is deprecated. It
+used to report whether memory for that aggregate had ever been
+allocated. This behavior may disappear in future versions of Perl.
+You should instead use a simple test for size:
+
+ if (@an_array) { print "has array elements\n" }
+ if (%a_hash) { print "has hash members\n" }
+
+When used on a hash element, it tells you whether the value is defined,
+not whether the key exists in the hash. Use L</exists> for the latter
+purpose.
+
+Examples:
+
+ print if defined $switch{D};
+ print "$val\n" while defined($val = pop(@ary));
+ die "Can't readlink $sym: $!"
+ unless defined($value = readlink $sym);
+ sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
+ $debugging = 0 unless defined $debugging;
+
+Note: Many folks tend to overuse C<defined> and are then surprised to
+discover that the number C<0> and C<""> (the zero-length string) are, in fact,
+defined values. For example, if you say
+
+ "ab" =~ /a(.*)b/;
+
+The pattern match succeeds and C<$1> is defined, although it
+matched "nothing". It didn't really fail to match anything. Rather, it
+matched something that happened to be zero characters long. This is all
+very above-board and honest. When a function returns an undefined value,
+it's an admission that it couldn't give you an honest answer. So you
+should use C<defined> only when questioning the integrity of what
+you're trying to do. At other times, a simple comparison to C<0> or C<""> is
+what you want.
+
+See also L</undef>, L</exists>, L</ref>.
+
+=item delete EXPR
+X<delete>
+
+Given an expression that specifies an element or slice of a hash, C<delete>
+deletes the specified elements from that hash so that exists() on that element
+no longer returns true. Setting a hash element to the undefined value does
+not remove its key, but deleting it does; see L</exists>.
+
+In list context, returns the value or values deleted, or the last such
+element in scalar context. The return list's length always matches that of
+the argument list: deleting non-existent elements returns the undefined value
+in their corresponding positions.
+
+delete() may also be used on arrays and array slices, but its behavior is less
+straightforward. Although exists() will return false for deleted entries,
+deleting array elements never changes indices of existing values; use shift()
+or splice() for that. However, if all deleted elements fall at the end of an
+array, the array's size shrinks to the position of the highest element that
+still tests true for exists(), or to 0 if none do.
+
+B<WARNING:> Calling delete on array values is deprecated and likely to
+be removed in a future version of Perl.
+
+Deleting from C<%ENV> modifies the environment. Deleting from a hash tied to
+a DBM file deletes the entry from the DBM file. Deleting from a C<tied> hash
+or array may not necessarily return anything; it depends on the implementation
+of the C<tied> package's DELETE method, which may do whatever it pleases.
+
+The C<delete local EXPR> construct localizes the deletion to the current
+block at run time. Until the block exits, elements locally deleted
+temporarily no longer exist. See L<perlsub/"Localized deletion of elements
+of composite types">.
+
+ %hash = (foo => 11, bar => 22, baz => 33);
+ $scalar = delete $hash{foo}; # $scalar is 11
+ $scalar = delete @hash{qw(foo bar)}; # $scalar is 22
+ @array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
+
+The following (inefficiently) deletes all the values of %HASH and @ARRAY:
+
+ foreach $key (keys %HASH) {
+ delete $HASH{$key};
+ }
+
+ foreach $index (0 .. $#ARRAY) {
+ delete $ARRAY[$index];
+ }
+
+And so do these:
+
+ delete @HASH{keys %HASH};
+
+ delete @ARRAY[0 .. $#ARRAY];
+
+But both are slower than assigning the empty list
+or undefining %HASH or @ARRAY, which is the customary
+way to empty out an aggregate:
+
+ %HASH = (); # completely empty %HASH
+ undef %HASH; # forget %HASH ever existed
+
+ @ARRAY = (); # completely empty @ARRAY
+ undef @ARRAY; # forget @ARRAY ever existed
+
+The EXPR can be arbitrarily complicated provided its
+final operation is an element or slice of an aggregate:
+
+ delete $ref->[$x][$y]{$key};
+ delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
+
+ delete $ref->[$x][$y][$index];
+ delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
+
+=item die LIST
+X<die> X<throw> X<exception> X<raise> X<$@> X<abort>
+
+C<die> raises an exception. Inside an C<eval> the error message is stuffed
+into C<$@> and the C<eval> is terminated with the undefined value.
+If the exception is outside of all enclosing C<eval>s, then the uncaught
+exception prints LIST to C<STDERR> and exits with a non-zero value. If you
+need to exit the process with a specific exit code, see L</exit>.
+
+Equivalent examples:
+
+ die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
+ chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
+
+If the last element of LIST does not end in a newline, the current
+script line number and input line number (if any) are also printed,
+and a newline is supplied. Note that the "input line number" (also
+known as "chunk") is subject to whatever notion of "line" happens to
+be currently in effect, and is also available as the special variable
+C<$.>. See L<perlvar/"$/"> and L<perlvar/"$.">.
+
+Hint: sometimes appending C<", stopped"> to your message will cause it
+to make better sense when the string C<"at foo line 123"> is appended.
+Suppose you are running script "canasta".
+
+ die "/etc/games is no good";
+ die "/etc/games is no good, stopped";
+
+produce, respectively
+
+ /etc/games is no good at canasta line 123.
+ /etc/games is no good, stopped at canasta line 123.
+
+If the output is empty and C<$@> already contains a value (typically from a
+previous eval) that value is reused after appending C<"\t...propagated">.
+This is useful for propagating exceptions:
+
+ eval { ... };
+ die unless $@ =~ /Expected exception/;
+
+If the output is empty and C<$@> contains an object reference that has a
+C<PROPAGATE> method, that method will be called with additional file
+and line number parameters. The return value replaces the value in
+C<$@>; i.e., as if C<< $@ = eval { $@->PROPAGATE(__FILE__, __LINE__) }; >>
+were called.
+
+If C<$@> is empty then the string C<"Died"> is used.
+
+If an uncaught exception results in interpreter exit, the exit code is
+determined from the values of C<$!> and C<$?> with this pseudocode:
+
+ exit $! if $!; # errno
+ exit $? >> 8 if $? >> 8; # child exit status
+ exit 255; # last resort
+
+The intent is to squeeze as much possible information about the likely cause
+into the limited space of the system exit code. However, as C<$!> is the value
+of C's C<errno>, which can be set by any system call, this means that the value
+of the exit code used by C<die> can be non-predictable, so should not be relied
+upon, other than to be non-zero.
+
+You can also call C<die> with a reference argument, and if this is trapped
+within an C<eval>, C<$@> contains that reference. This permits more
+elaborate exception handling using objects that maintain arbitrary state
+about the exception. Such a scheme is sometimes preferable to matching
+particular string values of C<$@> with regular expressions. Because C<$@>
+is a global variable and C<eval> may be used within object implementations,
+be careful that analyzing the error object doesn't replace the reference in
+the global variable. It's easiest to make a local copy of the reference
+before any manipulations. Here's an example:
+
+ use Scalar::Util "blessed";
+
+ eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
+ if (my $ev_err = $@) {
+ if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
+ # handle Some::Module::Exception
+ }
+ else {
+ # handle all other possible exceptions
+ }
+ }
+
+Because Perl stringifies uncaught exception messages before display,
+you'll probably want to overload stringification operations on
+exception objects. See L<overload> for details about that.
+
+You can arrange for a callback to be run just before the C<die>
+does its deed, by setting the C<$SIG{__DIE__}> hook. The associated
+handler is called with the error text and can change the error
+message, if it sees fit, by calling C<die> again. See
+L<perlvar/%SIG> for details on setting C<%SIG> entries, and
+L<"eval BLOCK"> for some examples. Although this feature was
+to be run only right before your program was to exit, this is not
+currently so: the C<$SIG{__DIE__}> hook is currently called
+even inside eval()ed blocks/strings! If one wants the hook to do
+nothing in such situations, put
+
+ die @_ if $^S;
+
+as the first line of the handler (see L<perlvar/$^S>). Because
+this promotes strange action at a distance, this counterintuitive
+behavior may be fixed in a future release.
+
+See also exit(), warn(), and the Carp module.
+
+=item do BLOCK
+X<do> X<block>
+
+Not really a function. Returns the value of the last command in the
+sequence of commands indicated by BLOCK. When modified by the C<while> or
+C<until> loop modifier, executes the BLOCK once before testing the loop
+condition. (On other statements the loop modifiers test the conditional
+first.)
+
+C<do BLOCK> does I<not> count as a loop, so the loop control statements
+C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
+See L<perlsyn> for alternative strategies.
+
+=item do SUBROUTINE(LIST)
+X<do>
+
+This form of subroutine call is deprecated. SUBROUTINE can be a bareword,
+a scalar variable or a subroutine beginning with C<&>.
+
+=item do EXPR
+X<do>
+
+Uses the value of EXPR as a filename and executes the contents of the
+file as a Perl script.
+
+ do 'stat.pl';
+
+is just like
+
+ eval `cat stat.pl`;
+
+except that it's more efficient and concise, keeps track of the current
+filename for error messages, searches the C<@INC> directories, and updates
+C<%INC> if the file is found. See L<perlvar/@INC> and L<perlvar/%INC> for
+these variables. It also differs in that code evaluated with C<do FILENAME>
+cannot see lexicals in the enclosing scope; C<eval STRING> does. It's the
+same, however, in that it does reparse the file every time you call it,
+so you probably don't want to do this inside a loop.
+
+If C<do> can read the file but cannot compile it, it returns C<undef> and sets
+an error message in C<$@>. If C<do> cannot read the file, it returns undef
+and sets C<$!> to the error. Always check C<$@> first, as compilation
+could fail in a way that also sets C<$!>. If the file is successfully
+compiled, C<do> returns the value of the last expression evaluated.
+
+Inclusion of library modules is better done with the
+C<use> and C<require> operators, which also do automatic error checking
+and raise an exception if there's a problem.
+
+You might like to use C<do> to read in a program configuration
+file. Manual error checking can be done this way:
+
+ # read in config files: system first, then user
+ for $file ("/share/prog/defaults.rc",
+ "$ENV{HOME}/.someprogrc")
+ {
+ unless ($return = do $file) {
+ warn "couldn't parse $file: $@" if $@;
+ warn "couldn't do $file: $!" unless defined $return;
+ warn "couldn't run $file" unless $return;
+ }
+ }
+
+=item dump LABEL
+X<dump> X<core> X<undump>
+
+=item dump
+
+This function causes an immediate core dump. See also the B<-u>
+command-line switch in L<perlrun>, which does the same thing.
+Primarily this is so that you can use the B<undump> program (not
+supplied) to turn your core dump into an executable binary after
+having initialized all your variables at the beginning of the
+program. When the new binary is executed it will begin by executing
+a C<goto LABEL> (with all the restrictions that C<goto> suffers).
+Think of it as a goto with an intervening core dump and reincarnation.
+If C<LABEL> is omitted, restarts the program from the top.
+
+B<WARNING>: Any files opened at the time of the dump will I<not>
+be open any more when the program is reincarnated, with possible
+resulting confusion by Perl.
+
+This function is now largely obsolete, mostly because it's very hard to
+convert a core file into an executable. That's why you should now invoke
+it as C<CORE::dump()>, if you don't want to be warned against a possible
+typo.
+
+Portability issues: L<perlport/dump>.
+
+=item each HASH
+X<each> X<hash, iterator>
+
+=item each ARRAY
+X<array, iterator>
+
+=item each EXPR
+
+When called in list context, returns a 2-element list consisting of the key
+and value for the next element of a hash, or the index and value for the
+next element of an array, so that you can iterate over it. When called in
+scalar context, returns only the key (not the value) in a hash, or the index
+in an array.
+
+Hash entries are returned in an apparently random order. The actual random
+order is subject to change in future versions of Perl, but it is
+guaranteed to be in the same order as either the C<keys> or C<values>
+function would produce on the same (unmodified) hash. Since Perl
+5.8.2 the ordering can be different even between different runs of Perl
+for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
+
+After C<each> has returned all entries from the hash or array, the next
+call to C<each> returns the empty list in list context and C<undef> in
+scalar context. The next call following that one restarts iteration. Each
+hash or array has its own internal iterator, accessed by C<each>, C<keys>,
+and C<values>. The iterator is implicitly reset when C<each> has reached
+the end as just described; it can be explicitly reset by calling C<keys> or
+C<values> on the hash or array. If you add or delete a hash's elements
+while iterating over it, entries may be skipped or duplicated--so don't do
+that. Exception: It is always safe to delete the item most recently
+returned by C<each()>, so the following code works properly:
+
+ while (($key, $value) = each %hash) {
+ print $key, "\n";
+ delete $hash{$key}; # This is safe
+ }
+
+This prints out your environment like the printenv(1) program,
+but in a different order:
+
+ while (($key,$value) = each %ENV) {
+ print "$key=$value\n";
+ }
+
+Starting with Perl 5.14, C<each> can take a scalar EXPR, which must hold
+reference to an unblessed hash or array. The argument will be dereferenced
+automatically. This aspect of C<each> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
+ while (($key,$value) = each $hashref) { ... }
+
+See also C<keys>, C<values>, and C<sort>.
+
+=item eof FILEHANDLE
+X<eof>
+X<end of file>
+X<end-of-file>
+
+=item eof ()
+
+=item eof
+
+Returns 1 if the next read on FILEHANDLE will return end of file I<or> if
+FILEHANDLE is not open. FILEHANDLE may be an expression whose value
+gives the real filehandle. (Note that this function actually
+reads a character and then C<ungetc>s it, so isn't useful in an
+interactive context.) Do not read from a terminal file (or call
+C<eof(FILEHANDLE)> on it) after end-of-file is reached. File types such
+as terminals may lose the end-of-file condition if you do.
+
+An C<eof> without an argument uses the last file read. Using C<eof()>
+with empty parentheses is different. It refers to the pseudo file
+formed from the files listed on the command line and accessed via the
+C<< <> >> operator. Since C<< <> >> isn't explicitly opened,
+as a normal filehandle is, an C<eof()> before C<< <> >> has been
+used will cause C<@ARGV> to be examined to determine if input is
+available. Similarly, an C<eof()> after C<< <> >> has returned
+end-of-file will assume you are processing another C<@ARGV> list,
+and if you haven't set C<@ARGV>, will read input from C<STDIN>;
+see L<perlop/"I/O Operators">.
+
+In a C<< while (<>) >> loop, C<eof> or C<eof(ARGV)> can be used to
+detect the end of each file, whereas C<eof()> will detect the end
+of the very last file only. Examples:
+
+ # reset line numbering on each input file
+ while (<>) {
+ next if /^\s*#/; # skip comments
+ print "$.\t$_";
+ } continue {
+ close ARGV if eof; # Not eof()!
+ }
+
+ # insert dashes just before last line of last file
+ while (<>) {
+ if (eof()) { # check for end of last file
+ print "--------------\n";
+ }
+ print;
+ last if eof(); # needed if we're reading from a terminal
+ }
+
+Practical hint: you almost never need to use C<eof> in Perl, because the
+input operators typically return C<undef> when they run out of data or
+encounter an error.
+
+=item eval EXPR
+X<eval> X<try> X<catch> X<evaluate> X<parse> X<execute>
+X<error, handling> X<exception, handling>
+
+=item eval BLOCK
+
+=item eval
+
+In the first form, the return value of EXPR is parsed and executed as if it
+were a little Perl program. The value of the expression (which is itself
+determined within scalar context) is first parsed, and if there were no
+errors, executed as a block within the lexical context of the current Perl
+program. This means, that in particular, any outer lexical variables are
+visible to it, and any package variable settings or subroutine and format
+definitions remain afterwards.
+
+Note that the value is parsed every time the C<eval> executes.
+If EXPR is omitted, evaluates C<$_>. This form is typically used to
+delay parsing and subsequent execution of the text of EXPR until run time.
+
+If the C<unicode_eval> feature is enabled (which is the default under a
+C<use 5.16> or higher declaration), EXPR or C<$_> is treated as a string of
+characters, so C<use utf8> declarations have no effect, and source filters
+are forbidden. In the absence of the C<unicode_eval> feature, the string
+will sometimes be treated as characters and sometimes as bytes, depending
+on the internal encoding, and source filters activated within the C<eval>
+exhibit the erratic, but historical, behaviour of affecting some outer file
+scope that is still compiling. See also the L</evalbytes> keyword, which
+always treats its input as a byte stream and works properly with source
+filters, and the L<feature> pragma.
+
+In the second form, the code within the BLOCK is parsed only once--at the
+same time the code surrounding the C<eval> itself was parsed--and executed
+within the context of the current Perl program. This form is typically
+used to trap exceptions more efficiently than the first (see below), while
+also providing the benefit of checking the code within BLOCK at compile
+time.
+
+The final semicolon, if any, may be omitted from the value of EXPR or within
+the BLOCK.
+
+In both forms, the value returned is the value of the last expression
+evaluated inside the mini-program; a return statement may be also used, just
+as with subroutines. The expression providing the return value is evaluated
+in void, scalar, or list context, depending on the context of the C<eval>
+itself. See L</wantarray> for more on how the evaluation context can be
+determined.
+
+If there is a syntax error or runtime error, or a C<die> statement is
+executed, C<eval> returns C<undef> in scalar context
+or an empty list--or, for syntax errors, a list containing a single
+undefined value--in list context, and C<$@> is set to the error
+message. The discrepancy in the return values in list context is
+considered a bug by some, and will probably be fixed in a future
+release. If there was no error, C<$@> is set to the empty string. A
+control flow operator like C<last> or C<goto> can bypass the setting of
+C<$@>. Beware that using C<eval> neither silences Perl from printing
+warnings to STDERR, nor does it stuff the text of warning messages into C<$@>.
+To do either of those, you have to use the C<$SIG{__WARN__}> facility, or
+turn off warnings inside the BLOCK or EXPR using S<C<no warnings 'all'>>.
+See L</warn>, L<perlvar>, L<warnings> and L<perllexwarn>.
+
+Note that, because C<eval> traps otherwise-fatal errors, it is useful for
+determining whether a particular feature (such as C<socket> or C<symlink>)
+is implemented. It is also Perl's exception-trapping mechanism, where
+the die operator is used to raise exceptions.
+
+If you want to trap errors when loading an XS module, some problems with
+the binary interface (such as Perl version skew) may be fatal even with
+C<eval> unless C<$ENV{PERL_DL_NONLAZY}> is set. See L<perlrun>.
+
+If the code to be executed doesn't vary, you may use the eval-BLOCK
+form to trap run-time errors without incurring the penalty of
+recompiling each time. The error, if any, is still returned in C<$@>.
+Examples:
+
+ # make divide-by-zero nonfatal
+ eval { $answer = $a / $b; }; warn $@ if $@;
+
+ # same thing, but less efficient
+ eval '$answer = $a / $b'; warn $@ if $@;
+
+ # a compile-time error
+ eval { $answer = }; # WRONG
+
+ # a run-time error
+ eval '$answer ='; # sets $@
+
+Using the C<eval{}> form as an exception trap in libraries does have some
+issues. Due to the current arguably broken state of C<__DIE__> hooks, you
+may wish not to trigger any C<__DIE__> hooks that user code may have installed.
+You can use the C<local $SIG{__DIE__}> construct for this purpose,
+as this example shows:
+
+ # a private exception trap for divide-by-zero
+ eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
+ warn $@ if $@;
+
+This is especially significant, given that C<__DIE__> hooks can call
+C<die> again, which has the effect of changing their error messages:
+
+ # __DIE__ hooks may modify error messages
+ {
+ local $SIG{'__DIE__'} =
+ sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
+ eval { die "foo lives here" };
+ print $@ if $@; # prints "bar lives here"
+ }
+
+Because this promotes action at a distance, this counterintuitive behavior
+may be fixed in a future release.
+
+With an C<eval>, you should be especially careful to remember what's
+being looked at when:
+
+ eval $x; # CASE 1
+ eval "$x"; # CASE 2
+
+ eval '$x'; # CASE 3
+ eval { $x }; # CASE 4
+
+ eval "\$$x++"; # CASE 5
+ $$x++; # CASE 6
+
+Cases 1 and 2 above behave identically: they run the code contained in
+the variable $x. (Although case 2 has misleading double quotes making
+the reader wonder what else might be happening (nothing is).) Cases 3
+and 4 likewise behave in the same way: they run the code C<'$x'>, which
+does nothing but return the value of $x. (Case 4 is preferred for
+purely visual reasons, but it also has the advantage of compiling at
+compile-time instead of at run-time.) Case 5 is a place where
+normally you I<would> like to use double quotes, except that in this
+particular situation, you can just use symbolic references instead, as
+in case 6.
+
+Before Perl 5.14, the assignment to C<$@> occurred before restoration
+of localised variables, which means that for your code to run on older
+versions, a temporary is required if you want to mask some but not all
+errors:
+
+ # alter $@ on nefarious repugnancy only
+ {
+ my $e;
+ {
+ local $@; # protect existing $@
+ eval { test_repugnancy() };
+ # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
+ $@ =~ /nefarious/ and $e = $@;
+ }
+ die $e if defined $e
+ }
+
+C<eval BLOCK> does I<not> count as a loop, so the loop control statements
+C<next>, C<last>, or C<redo> cannot be used to leave or restart the block.
+
+An C<eval ''> executed within the C<DB> package doesn't see the usual
+surrounding lexical scope, but rather the scope of the first non-DB piece
+of code that called it. You don't normally need to worry about this unless
+you are writing a Perl debugger.
+
+=item evalbytes EXPR
+X<evalbytes>
+
+=item evalbytes
+
+This function is like L</eval> with a string argument, except it always
+parses its argument, or C<$_> if EXPR is omitted, as a string of bytes. A
+string containing characters whose ordinal value exceeds 255 results in an
+error. Source filters activated within the evaluated code apply to the
+code itself.
+
+This function is only available under the C<evalbytes> feature, a
+C<use v5.16> (or higher) declaration, or with a C<CORE::> prefix. See
+L<feature> for more information.
+
+=item exec LIST
+X<exec> X<execute>
+
+=item exec PROGRAM LIST
+
+The C<exec> function executes a system command I<and never returns>;
+use C<system> instead of C<exec> if you want it to return. It fails and
+returns false only if the command does not exist I<and> it is executed
+directly instead of via your system's command shell (see below).
+
+Since it's a common mistake to use C<exec> instead of C<system>, Perl
+warns you if there is a following statement that isn't C<die>, C<warn>,
+or C<exit> (if C<-w> is set--but you always do that, right?). If you
+I<really> want to follow an C<exec> with some other statement, you
+can use one of these styles to avoid the warning:
+
+ exec ('foo') or print STDERR "couldn't exec foo: $!";
+ { exec ('foo') }; print STDERR "couldn't exec foo: $!";
+
+If there is more than one argument in LIST, or if LIST is an array
+with more than one value, calls execvp(3) with the arguments in LIST.
+If there is only one scalar argument or an array with one element in it,
+the argument is checked for shell metacharacters, and if there are any,
+the entire argument is passed to the system's command shell for parsing
+(this is C</bin/sh -c> on Unix platforms, but varies on other platforms).
+If there are no shell metacharacters in the argument, it is split into
+words and passed directly to C<execvp>, which is more efficient.
+Examples:
+
+ exec '/bin/echo', 'Your arguments are: ', @ARGV;
+ exec "sort $outfile | uniq";
+
+If you don't really want to execute the first argument, but want to lie
+to the program you are executing about its own name, you can specify
+the program you actually want to run as an "indirect object" (without a
+comma) in front of the LIST. (This always forces interpretation of the
+LIST as a multivalued list, even if there is only a single scalar in
+the list.) Example:
+
+ $shell = '/bin/csh';
+ exec $shell '-sh'; # pretend it's a login shell
+
+or, more directly,
+
+ exec {'/bin/csh'} '-sh'; # pretend it's a login shell
+
+When the arguments get executed via the system shell, results are
+subject to its quirks and capabilities. See L<perlop/"`STRING`">
+for details.
+
+Using an indirect object with C<exec> or C<system> is also more
+secure. This usage (which also works fine with system()) forces
+interpretation of the arguments as a multivalued list, even if the
+list had just one argument. That way you're safe from the shell
+expanding wildcards or splitting up words with whitespace in them.
+
+ @args = ( "echo surprise" );
+
+ exec @args; # subject to shell escapes
+ # if @args == 1
+ exec { $args[0] } @args; # safe even with one-arg list
+
+The first version, the one without the indirect object, ran the I<echo>
+program, passing it C<"surprise"> an argument. The second version didn't;
+it tried to run a program named I<"echo surprise">, didn't find it, and set
+C<$?> to a non-zero value indicating failure.
+
+Beginning with v5.6.0, Perl attempts to flush all files opened for
+output before the exec, but this may not be supported on some platforms
+(see L<perlport>). To be safe, you may need to set C<$|> ($AUTOFLUSH
+in English) or call the C<autoflush()> method of C<IO::Handle> on any
+open handles to avoid lost output.
+
+Note that C<exec> will not call your C<END> blocks, nor will it invoke
+C<DESTROY> methods on your objects.
+
+Portability issues: L<perlport/exec>.
+
+=item exists EXPR
+X<exists> X<autovivification>
+
+Given an expression that specifies an element of a hash, returns true if the
+specified element in the hash has ever been initialized, even if the
+corresponding value is undefined.
+
+ print "Exists\n" if exists $hash{$key};
+ print "Defined\n" if defined $hash{$key};
+ print "True\n" if $hash{$key};
+
+exists may also be called on array elements, but its behavior is much less
+obvious and is strongly tied to the use of L</delete> on arrays. B<Be aware>
+that calling exists on array values is deprecated and likely to be removed in
+a future version of Perl.
+
+ print "Exists\n" if exists $array[$index];
+ print "Defined\n" if defined $array[$index];
+ print "True\n" if $array[$index];
+
+A hash or array element can be true only if it's defined and defined only if
+it exists, but the reverse doesn't necessarily hold true.
+
+Given an expression that specifies the name of a subroutine,
+returns true if the specified subroutine has ever been declared, even
+if it is undefined. Mentioning a subroutine name for exists or defined
+does not count as declaring it. Note that a subroutine that does not
+exist may still be callable: its package may have an C<AUTOLOAD>
+method that makes it spring into existence the first time that it is
+called; see L<perlsub>.
+
+ print "Exists\n" if exists &subroutine;
+ print "Defined\n" if defined &subroutine;
+
+Note that the EXPR can be arbitrarily complicated as long as the final
+operation is a hash or array key lookup or subroutine name:
+
+ if (exists $ref->{A}->{B}->{$key}) { }
+ if (exists $hash{A}{B}{$key}) { }
+
+ if (exists $ref->{A}->{B}->[$ix]) { }
+ if (exists $hash{A}{B}[$ix]) { }
+
+ if (exists &{$ref->{A}{B}{$key}}) { }
+
+Although the mostly deeply nested array or hash will not spring into
+existence just because its existence was tested, any intervening ones will.
+Thus C<< $ref->{"A"} >> and C<< $ref->{"A"}->{"B"} >> will spring
+into existence due to the existence test for the $key element above.
+This happens anywhere the arrow operator is used, including even here:
+
+ undef $ref;
+ if (exists $ref->{"Some key"}) { }
+ print $ref; # prints HASH(0x80d3d5c)
+
+This surprising autovivification in what does not at first--or even
+second--glance appear to be an lvalue context may be fixed in a future
+release.
+
+Use of a subroutine call, rather than a subroutine name, as an argument
+to exists() is an error.
+
+ exists ⊂ # OK
+ exists &sub(); # Error
+
+=item exit EXPR
+X<exit> X<terminate> X<abort>
+
+=item exit
+
+Evaluates EXPR and exits immediately with that value. Example:
+
+ $ans = <STDIN>;
+ exit 0 if $ans =~ /^[Xx]/;
+
+See also C<die>. If EXPR is omitted, exits with C<0> status. The only
+universally recognized values for EXPR are C<0> for success and C<1>
+for error; other values are subject to interpretation depending on the
+environment in which the Perl program is running. For example, exiting
+69 (EX_UNAVAILABLE) from a I<sendmail> incoming-mail filter will cause
+the mailer to return the item undelivered, but that's not true everywhere.
+
+Don't use C<exit> to abort a subroutine if there's any chance that
+someone might want to trap whatever error happened. Use C<die> instead,
+which can be trapped by an C<eval>.
+
+The exit() function does not always exit immediately. It calls any
+defined C<END> routines first, but these C<END> routines may not
+themselves abort the exit. Likewise any object destructors that need to
+be called are called before the real exit. C<END> routines and destructors
+can change the exit status by modifying C<$?>. If this is a problem, you
+can call C<POSIX:_exit($status)> to avoid END and destructor processing.
+See L<perlmod> for details.
+
+Portability issues: L<perlport/exit>.
+
+=item exp EXPR
+X<exp> X<exponential> X<antilog> X<antilogarithm> X<e>
+
+=item exp
+
+Returns I<e> (the natural logarithm base) to the power of EXPR.
+If EXPR is omitted, gives C<exp($_)>.
+
+=item fcntl FILEHANDLE,FUNCTION,SCALAR
+X<fcntl>
+
+Implements the fcntl(2) function. You'll probably have to say
+
+ use Fcntl;
+
+first to get the correct constant definitions. Argument processing and
+value returned work just like C<ioctl> below.
+For example:
+
+ use Fcntl;
+ fcntl($filehandle, F_GETFL, $packed_return_buffer)
+ or die "can't fcntl F_GETFL: $!";
+
+You don't have to check for C<defined> on the return from C<fcntl>.
+Like C<ioctl>, it maps a C<0> return from the system call into
+C<"0 but true"> in Perl. This string is true in boolean context and C<0>
+in numeric context. It is also exempt from the normal B<-w> warnings
+on improper numeric conversions.
+
+Note that C<fcntl> raises an exception if used on a machine that
+doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
+manpage to learn what functions are available on your system.
+
+Here's an example of setting a filehandle named C<REMOTE> to be
+non-blocking at the system level. You'll have to negotiate C<$|>
+on your own, though.
+
+ use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
+
+ $flags = fcntl(REMOTE, F_GETFL, 0)
+ or die "Can't get flags for the socket: $!\n";
+
+ $flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
+ or die "Can't set flags for the socket: $!\n";
+
+Portability issues: L<perlport/fcntl>.
+
+=item __FILE__
+X<__FILE__>
+
+A special token that returns the name of the file in which it occurs.
+
+=item fileno FILEHANDLE
+X<fileno>
+
+Returns the file descriptor for a filehandle, or undefined if the
+filehandle is not open. If there is no real file descriptor at the OS
+level, as can happen with filehandles connected to memory objects via
+C<open> with a reference for the third argument, -1 is returned.
+
+This is mainly useful for constructing
+bitmaps for C<select> and low-level POSIX tty-handling operations.
+If FILEHANDLE is an expression, the value is taken as an indirect
+filehandle, generally its name.
+
+You can use this to find out whether two handles refer to the
+same underlying descriptor:
+
+ if (fileno(THIS) == fileno(THAT)) {
+ print "THIS and THAT are dups\n";
+ }
+
+=item flock FILEHANDLE,OPERATION
+X<flock> X<lock> X<locking>
+
+Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
+for success, false on failure. Produces a fatal error if used on a
+machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
+C<flock> is Perl's portable file-locking interface, although it locks
+entire files only, not records.
+
+Two potentially non-obvious but traditional C<flock> semantics are
+that it waits indefinitely until the lock is granted, and that its locks
+are B<merely advisory>. Such discretionary locks are more flexible, but
+offer fewer guarantees. This means that programs that do not also use
+C<flock> may modify files locked with C<flock>. See L<perlport>,
+your port's specific documentation, and your system-specific local manpages
+for details. It's best to assume traditional behavior if you're writing
+portable programs. (But if you're not, you should as always feel perfectly
+free to write for your own system's idiosyncrasies (sometimes called
+"features"). Slavish adherence to portability concerns shouldn't get
+in the way of your getting your job done.)
+
+OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
+LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
+you can use the symbolic names if you import them from the L<Fcntl> module,
+either individually, or as a group using the C<:flock> tag. LOCK_SH
+requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
+releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
+LOCK_SH or LOCK_EX, then C<flock> returns immediately rather than blocking
+waiting for the lock; check the return status to see if you got it.
+
+To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
+before locking or unlocking it.
+
+Note that the emulation built with lockf(3) doesn't provide shared
+locks, and it requires that FILEHANDLE be open with write intent. These
+are the semantics that lockf(3) implements. Most if not all systems
+implement lockf(3) in terms of fcntl(2) locking, though, so the
+differing semantics shouldn't bite too many people.
+
+Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
+be open with read intent to use LOCK_SH and requires that it be open
+with write intent to use LOCK_EX.
+
+Note also that some versions of C<flock> cannot lock things over the
+network; you would need to use the more system-specific C<fcntl> for
+that. If you like you can force Perl to ignore your system's flock(2)
+function, and so provide its own fcntl(2)-based emulation, by passing
+the switch C<-Ud_flock> to the F<Configure> program when you configure
+and build a new Perl.
+
+Here's a mailbox appender for BSD systems.
+
+ use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
+
+ sub lock {
+ my ($fh) = @_;
+ flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
+
+ # and, in case someone appended while we were waiting...
+ seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
+ }
+
+ sub unlock {
+ my ($fh) = @_;
+ flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
+ }
+
+ open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
+ or die "Can't open mailbox: $!";
+
+ lock($mbox);
+ print $mbox $msg,"\n\n";
+ unlock($mbox);
+
+On systems that support a real flock(2), locks are inherited across fork()
+calls, whereas those that must resort to the more capricious fcntl(2)
+function lose their locks, making it seriously harder to write servers.
+
+See also L<DB_File> for other flock() examples.
+
+Portability issues: L<perlport/flock>.
+
+=item fork
+X<fork> X<child> X<parent>
+
+Does a fork(2) system call to create a new process running the
+same program at the same point. It returns the child pid to the
+parent process, C<0> to the child process, or C<undef> if the fork is
+unsuccessful. File descriptors (and sometimes locks on those descriptors)
+are shared, while everything else is copied. On most systems supporting
+fork(), great care has gone into making it extremely efficient (for
+example, using copy-on-write technology on data pages), making it the
+dominant paradigm for multitasking over the last few decades.
+
+Beginning with v5.6.0, Perl attempts to flush all files opened for
+output before forking the child process, but this may not be supported
+on some platforms (see L<perlport>). To be safe, you may need to set
+C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method of
+C<IO::Handle> on any open handles to avoid duplicate output.
+
+If you C<fork> without ever waiting on your children, you will
+accumulate zombies. On some systems, you can avoid this by setting
+C<$SIG{CHLD}> to C<"IGNORE">. See also L<perlipc> for more examples of
+forking and reaping moribund children.
+
+Note that if your forked child inherits system file descriptors like
+STDIN and STDOUT that are actually connected by a pipe or socket, even
+if you exit, then the remote server (such as, say, a CGI script or a
+backgrounded job launched from a remote shell) won't think you're done.
+You should reopen those to F</dev/null> if it's any issue.
+
+On some platforms such as Windows, where the fork() system call is not available,
+Perl can be built to emulate fork() in the Perl interpreter. The emulation is designed to,
+at the level of the Perl program, be as compatible as possible with the "Unix" fork().
+However it has limitations that have to be considered in code intended to be portable.
+See L<perlfork> for more details.
+
+Portability issues: L<perlport/fork>.
+
+=item format
+X<format>
+
+Declare a picture format for use by the C<write> function. For
+example:
+
+ format Something =
+ Test: @<<<<<<<< @||||| @>>>>>
+ $str, $%, '$' . int($num)
+ .
+
+ $str = "widget";
+ $num = $cost/$quantity;
+ $~ = 'Something';
+ write;
+
+See L<perlform> for many details and examples.
+
+=item formline PICTURE,LIST
+X<formline>
+
+This is an internal function used by C<format>s, though you may call it,
+too. It formats (see L<perlform>) a list of values according to the
+contents of PICTURE, placing the output into the format output
+accumulator, C<$^A> (or C<$ACCUMULATOR> in English).
+Eventually, when a C<write> is done, the contents of
+C<$^A> are written to some filehandle. You could also read C<$^A>
+and then set C<$^A> back to C<"">. Note that a format typically
+does one C<formline> per line of form, but the C<formline> function itself
+doesn't care how many newlines are embedded in the PICTURE. This means
+that the C<~> and C<~~> tokens treat the entire PICTURE as a single line.
+You may therefore need to use multiple formlines to implement a single
+record format, just like the C<format> compiler.
+
+Be careful if you put double quotes around the picture, because an C<@>
+character may be taken to mean the beginning of an array name.
+C<formline> always returns true. See L<perlform> for other examples.
+
+If you are trying to use this instead of C<write> to capture the output,
+you may find it easier to open a filehandle to a scalar
+(C<< open $fh, ">", \$output >>) and write to that instead.
+
+=item getc FILEHANDLE
+X<getc> X<getchar> X<character> X<file, read>
+
+=item getc
+
+Returns the next character from the input file attached to FILEHANDLE,
+or the undefined value at end of file or if there was an error (in
+the latter case C<$!> is set). If FILEHANDLE is omitted, reads from
+STDIN. This is not particularly efficient. However, it cannot be
+used by itself to fetch single characters without waiting for the user
+to hit enter. For that, try something more like:
+
+ if ($BSD_STYLE) {
+ system "stty cbreak </dev/tty >/dev/tty 2>&1";
+ }
+ else {
+ system "stty", '-icanon', 'eol', "\001";
+ }
+
+ $key = getc(STDIN);
+
+ if ($BSD_STYLE) {
+ system "stty -cbreak </dev/tty >/dev/tty 2>&1";
+ }
+ else {
+ system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
+ }
+ print "\n";
+
+Determination of whether $BSD_STYLE should be set
+is left as an exercise to the reader.
+
+The C<POSIX::getattr> function can do this more portably on
+systems purporting POSIX compliance. See also the C<Term::ReadKey>
+module from your nearest CPAN site; details on CPAN can be found under
+L<perlmodlib/CPAN>.
+
+=item getlogin
+X<getlogin> X<login>
+
+This implements the C library function of the same name, which on most
+systems returns the current login from F</etc/utmp>, if any. If it
+returns the empty string, use C<getpwuid>.
+
+ $login = getlogin || getpwuid($<) || "Kilroy";
+
+Do not consider C<getlogin> for authentication: it is not as
+secure as C<getpwuid>.
+
+Portability issues: L<perlport/getlogin>.
+
+=item getpeername SOCKET
+X<getpeername> X<peer>
+
+Returns the packed sockaddr address of the other end of the SOCKET
+connection.
+
+ use Socket;
+ $hersockaddr = getpeername(SOCK);
+ ($port, $iaddr) = sockaddr_in($hersockaddr);
+ $herhostname = gethostbyaddr($iaddr, AF_INET);
+ $herstraddr = inet_ntoa($iaddr);
+
+=item getpgrp PID
+X<getpgrp> X<group>
+
+Returns the current process group for the specified PID. Use
+a PID of C<0> to get the current process group for the
+current process. Will raise an exception if used on a machine that
+doesn't implement getpgrp(2). If PID is omitted, returns the process
+group of the current process. Note that the POSIX version of C<getpgrp>
+does not accept a PID argument, so only C<PID==0> is truly portable.
+
+Portability issues: L<perlport/getpgrp>.
+
+=item getppid
+X<getppid> X<parent> X<pid>
+
+Returns the process id of the parent process.
+
+Note for Linux users: on Linux, the C functions C<getpid()> and
+C<getppid()> return different values from different threads. In order to
+be portable, this behavior is not reflected by the Perl-level function
+C<getppid()>, that returns a consistent value across threads. If you want
+to call the underlying C<getppid()>, you may use the CPAN module
+C<Linux::Pid>.
+
+Portability issues: L<perlport/getppid>.
+
+=item getpriority WHICH,WHO
+X<getpriority> X<priority> X<nice>
+
+Returns the current priority for a process, a process group, or a user.
+(See L<getpriority(2)>.) Will raise a fatal exception if used on a
+machine that doesn't implement getpriority(2).
+
+Portability issues: L<perlport/getpriority>.
+
+=item getpwnam NAME
+X<getpwnam> X<getgrnam> X<gethostbyname> X<getnetbyname> X<getprotobyname>
+X<getpwuid> X<getgrgid> X<getservbyname> X<gethostbyaddr> X<getnetbyaddr>
+X<getprotobynumber> X<getservbyport> X<getpwent> X<getgrent> X<gethostent>
+X<getnetent> X<getprotoent> X<getservent> X<setpwent> X<setgrent> X<sethostent>
+X<setnetent> X<setprotoent> X<setservent> X<endpwent> X<endgrent> X<endhostent>
+X<endnetent> X<endprotoent> X<endservent>
+
+=item getgrnam NAME
+
+=item gethostbyname NAME
+
+=item getnetbyname NAME
+
+=item getprotobyname NAME
+
+=item getpwuid UID
+
+=item getgrgid GID
+
+=item getservbyname NAME,PROTO
+
+=item gethostbyaddr ADDR,ADDRTYPE
+
+=item getnetbyaddr ADDR,ADDRTYPE
+
+=item getprotobynumber NUMBER
+
+=item getservbyport PORT,PROTO
+
+=item getpwent
+
+=item getgrent
+
+=item gethostent
+
+=item getnetent
+
+=item getprotoent
+
+=item getservent
+
+=item setpwent
+
+=item setgrent
+
+=item sethostent STAYOPEN
+
+=item setnetent STAYOPEN
+
+=item setprotoent STAYOPEN
+
+=item setservent STAYOPEN
+
+=item endpwent
+
+=item endgrent
+
+=item endhostent
+
+=item endnetent
+
+=item endprotoent
+
+=item endservent
+
+These routines are the same as their counterparts in the
+system C library. In list context, the return values from the
+various get routines are as follows:
+
+ ($name,$passwd,$uid,$gid,
+ $quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
+ ($name,$passwd,$gid,$members) = getgr*
+ ($name,$aliases,$addrtype,$length,@addrs) = gethost*
+ ($name,$aliases,$addrtype,$net) = getnet*
+ ($name,$aliases,$proto) = getproto*
+ ($name,$aliases,$port,$proto) = getserv*
+
+(If the entry doesn't exist you get an empty list.)
+
+The exact meaning of the $gcos field varies but it usually contains
+the real name of the user (as opposed to the login name) and other
+information pertaining to the user. Beware, however, that in many
+system users are able to change this information and therefore it
+cannot be trusted and therefore the $gcos is tainted (see
+L<perlsec>). The $passwd and $shell, user's encrypted password and
+login shell, are also tainted, for the same reason.
+
+In scalar context, you get the name, unless the function was a
+lookup by name, in which case you get the other thing, whatever it is.
+(If the entry doesn't exist you get the undefined value.) For example:
+
+ $uid = getpwnam($name);
+ $name = getpwuid($num);
+ $name = getpwent();
+ $gid = getgrnam($name);
+ $name = getgrgid($num);
+ $name = getgrent();
+ #etc.
+
+In I<getpw*()> the fields $quota, $comment, and $expire are special
+in that they are unsupported on many systems. If the
+$quota is unsupported, it is an empty scalar. If it is supported, it
+usually encodes the disk quota. If the $comment field is unsupported,
+it is an empty scalar. If it is supported it usually encodes some
+administrative comment about the user. In some systems the $quota
+field may be $change or $age, fields that have to do with password
+aging. In some systems the $comment field may be $class. The $expire
+field, if present, encodes the expiration period of the account or the
+password. For the availability and the exact meaning of these fields
+in your system, please consult getpwnam(3) and your system's
+F<pwd.h> file. You can also find out from within Perl what your
+$quota and $comment fields mean and whether you have the $expire field
+by using the C<Config> module and the values C<d_pwquota>, C<d_pwage>,
+C<d_pwchange>, C<d_pwcomment>, and C<d_pwexpire>. Shadow password
+files are supported only if your vendor has implemented them in the
+intuitive fashion that calling the regular C library routines gets the
+shadow versions if you're running under privilege or if there exists
+the shadow(3) functions as found in System V (this includes Solaris
+and Linux). Those systems that implement a proprietary shadow password
+facility are unlikely to be supported.
+
+The $members value returned by I<getgr*()> is a space-separated list of
+the login names of the members of the group.
+
+For the I<gethost*()> functions, if the C<h_errno> variable is supported in
+C, it will be returned to you via C<$?> if the function call fails. The
+C<@addrs> value returned by a successful call is a list of raw
+addresses returned by the corresponding library call. In the
+Internet domain, each address is four bytes long; you can unpack it
+by saying something like:
+
+ ($a,$b,$c,$d) = unpack('W4',$addr[0]);
+
+The Socket library makes this slightly easier:
+
+ use Socket;
+ $iaddr = inet_aton("127.1"); # or whatever address
+ $name = gethostbyaddr($iaddr, AF_INET);
+
+ # or going the other way
+ $straddr = inet_ntoa($iaddr);
+
+In the opposite way, to resolve a hostname to the IP address
+you can write this:
+
+ use Socket;
+ $packed_ip = gethostbyname("www.perl.org");
+ if (defined $packed_ip) {
+ $ip_address = inet_ntoa($packed_ip);
+ }
+
+Make sure C<gethostbyname()> is called in SCALAR context and that
+its return value is checked for definedness.
+
+The C<getprotobynumber> function, even though it only takes one argument,
+has the precedence of a list operator, so beware:
+
+ getprotobynumber $number eq 'icmp' # WRONG
+ getprotobynumber($number eq 'icmp') # actually means this
+ getprotobynumber($number) eq 'icmp' # better this way
+
+If you get tired of remembering which element of the return list
+contains which return value, by-name interfaces are provided
+in standard modules: C<File::stat>, C<Net::hostent>, C<Net::netent>,
+C<Net::protoent>, C<Net::servent>, C<Time::gmtime>, C<Time::localtime>,
+and C<User::grent>. These override the normal built-ins, supplying
+versions that return objects with the appropriate names
+for each field. For example:
+
+ use File::stat;
+ use User::pwent;
+ $is_his = (stat($filename)->uid == pwent($whoever)->uid);
+
+Even though it looks as though they're the same method calls (uid),
+they aren't, because a C<File::stat> object is different from
+a C<User::pwent> object.
+
+Portability issues: L<perlport/getpwnam> to L<perlport/endservent>.
+
+=item getsockname SOCKET
+X<getsockname>
+
+Returns the packed sockaddr address of this end of the SOCKET connection,
+in case you don't know the address because you have several different
+IPs that the connection might have come in on.
+
+ use Socket;
+ $mysockaddr = getsockname(SOCK);
+ ($port, $myaddr) = sockaddr_in($mysockaddr);
+ printf "Connect to %s [%s]\n",
+ scalar gethostbyaddr($myaddr, AF_INET),
+ inet_ntoa($myaddr);
+
+=item getsockopt SOCKET,LEVEL,OPTNAME
+X<getsockopt>
+
+Queries the option named OPTNAME associated with SOCKET at a given LEVEL.
+Options may exist at multiple protocol levels depending on the socket
+type, but at least the uppermost socket level SOL_SOCKET (defined in the
+C<Socket> module) will exist. To query options at another level the
+protocol number of the appropriate protocol controlling the option
+should be supplied. For example, to indicate that an option is to be
+interpreted by the TCP protocol, LEVEL should be set to the protocol
+number of TCP, which you can get using C<getprotobyname>.
+
+The function returns a packed string representing the requested socket
+option, or C<undef> on error, with the reason for the error placed in
+C<$!>. Just what is in the packed string depends on LEVEL and OPTNAME;
+consult getsockopt(2) for details. A common case is that the option is an
+integer, in which case the result is a packed integer, which you can decode
+using C<unpack> with the C<i> (or C<I>) format.
+
+Here's an example to test whether Nagle's algorithm is enabled on a socket:
+
+ use Socket qw(:all);
+
+ defined(my $tcp = getprotobyname("tcp"))
+ or die "Could not determine the protocol number for tcp";
+ # my $tcp = IPPROTO_TCP; # Alternative
+ my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
+ or die "getsockopt TCP_NODELAY: $!";
+ my $nodelay = unpack("I", $packed);
+ print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
+
+Portability issues: L<perlport/getsockopt>.
+
+=item given EXPR BLOCK
+X<given>
+
+=item given BLOCK
+
+C<given> is analogous to the C<switch> keyword in other languages. C<given>
+and C<when> are used in Perl to implement C<switch>/C<case> like statements.
+Only available after Perl 5.10. For example:
+
+ use v5.10;
+ given ($fruit) {
+ when (/apples?/) {
+ print "I like apples."
+ }
+ when (/oranges?/) {
+ print "I don't like oranges."
+ }
+ default {
+ print "I don't like anything"
+ }
+ }
+
+See L<perlsyn/"Switch statements"> for detailed information.
+
+=item glob EXPR
+X<glob> X<wildcard> X<filename, expansion> X<expand>
+
+=item glob
+
+In list context, returns a (possibly empty) list of filename expansions on
+the value of EXPR such as the standard Unix shell F</bin/csh> would do. In
+scalar context, glob iterates through such filename expansions, returning
+undef when the list is exhausted. This is the internal function
+implementing the C<< <*.c> >> operator, but you can use it directly. If
+EXPR is omitted, C<$_> is used. The C<< <*.c> >> operator is discussed in
+more detail in L<perlop/"I/O Operators">.
+
+Note that C<glob> splits its arguments on whitespace and treats
+each segment as separate pattern. As such, C<glob("*.c *.h")>
+matches all files with a F<.c> or F<.h> extension. The expression
+C<glob(".* *")> matches all files in the current working directory.
+If you want to glob filenames that might contain whitespace, you'll
+have to use extra quotes around the spacey filename to protect it.
+For example, to glob filenames that have an C<e> followed by a space
+followed by an C<f>, use either of:
+
+ @spacies = <"*e f*">;
+ @spacies = glob '"*e f*"';
+ @spacies = glob q("*e f*");
+
+If you had to get a variable through, you could do this:
+
+ @spacies = glob "'*${var}e f*'";
+ @spacies = glob qq("*${var}e f*");
+
+If non-empty braces are the only wildcard characters used in the
+C<glob>, no filenames are matched, but potentially many strings
+are returned. For example, this produces nine strings, one for
+each pairing of fruits and colors:
+
+ @many = glob "{apple,tomato,cherry}={green,yellow,red}";
+
+Beginning with v5.6.0, this operator is implemented using the standard
+C<File::Glob> extension. See L<File::Glob> for details, including
+C<bsd_glob> which does not treat whitespace as a pattern separator.
+
+Portability issues: L<perlport/glob>.
+
+=item gmtime EXPR
+X<gmtime> X<UTC> X<Greenwich>
+
+=item gmtime
+
+Works just like L</localtime> but the returned values are
+localized for the standard Greenwich time zone.
+
+Note: When called in list context, $isdst, the last value
+returned by gmtime, is always C<0>. There is no
+Daylight Saving Time in GMT.
+
+Portability issues: L<perlport/gmtime>.
+
+=item goto LABEL
+X<goto> X<jump> X<jmp>
+
+=item goto EXPR
+
+=item goto &NAME
+
+The C<goto-LABEL> form finds the statement labeled with LABEL and
+resumes execution there. It can't be used to get out of a block or
+subroutine given to C<sort>. It can be used to go almost anywhere
+else within the dynamic scope, including out of subroutines, but it's
+usually better to use some other construct such as C<last> or C<die>.
+The author of Perl has never felt the need to use this form of C<goto>
+(in Perl, that is; C is another matter). (The difference is that C
+does not offer named loops combined with loop control. Perl does, and
+this replaces most structured uses of C<goto> in other languages.)
+
+The C<goto-EXPR> form expects a label name, whose scope will be resolved
+dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
+necessarily recommended if you're optimizing for maintainability:
+
+ goto ("FOO", "BAR", "GLARCH")[$i];
+
+As shown in this example, C<goto-EXPR> is exempt from the "looks like a
+function" rule. A pair of parentheses following it does not (necessarily)
+delimit its argument. C<goto("NE")."XT"> is equivalent to C<goto NEXT>.
+
+Use of C<goto-LABEL> or C<goto-EXPR> to jump into a construct is
+deprecated and will issue a warning. Even then, it may not be used to
+go into any construct that requires initialization, such as a
+subroutine or a C<foreach> loop. It also can't be used to go into a
+construct that is optimized away.
+
+The C<goto-&NAME> form is quite different from the other forms of
+C<goto>. In fact, it isn't a goto in the normal sense at all, and
+doesn't have the stigma associated with other gotos. Instead, it
+exits the current subroutine (losing any changes set by local()) and
+immediately calls in its place the named subroutine using the current
+value of @_. This is used by C<AUTOLOAD> subroutines that wish to
+load another subroutine and then pretend that the other subroutine had
+been called in the first place (except that any modifications to C<@_>
+in the current subroutine are propagated to the other subroutine.)
+After the C<goto>, not even C<caller> will be able to tell that this
+routine was called first.
+
+NAME needn't be the name of a subroutine; it can be a scalar variable
+containing a code reference or a block that evaluates to a code
+reference.
+
+=item grep BLOCK LIST
+X<grep>
+
+=item grep EXPR,LIST
+
+This is similar in spirit to, but not the same as, grep(1) and its
+relatives. In particular, it is not limited to using regular expressions.
+
+Evaluates the BLOCK or EXPR for each element of LIST (locally setting
+C<$_> to each element) and returns the list value consisting of those
+elements for which the expression evaluated to true. In scalar
+context, returns the number of times the expression was true.
+
+ @foo = grep(!/^#/, @bar); # weed out comments
+
+or equivalently,
+
+ @foo = grep {!/^#/} @bar; # weed out comments
+
+Note that C<$_> is an alias to the list value, so it can be used to
+modify the elements of the LIST. While this is useful and supported,
+it can cause bizarre results if the elements of LIST are not variables.
+Similarly, grep returns aliases into the original list, much as a for
+loop's index variable aliases the list elements. That is, modifying an
+element of a list returned by grep (for example, in a C<foreach>, C<map>
+or another C<grep>) actually modifies the element in the original list.
+This is usually something to be avoided when writing clear code.
+
+If C<$_> is lexical in the scope where the C<grep> appears (because it has
+been declared with C<my $_>) then, in addition to being locally aliased to
+the list elements, C<$_> keeps being lexical inside the block; i.e., it
+can't be seen from the outside, avoiding any potential side-effects.
+
+See also L</map> for a list composed of the results of the BLOCK or EXPR.
+
+=item hex EXPR
+X<hex> X<hexadecimal>
+
+=item hex
+
+Interprets EXPR as a hex string and returns the corresponding value.
+(To convert strings that might start with either C<0>, C<0x>, or C<0b>, see
+L</oct>.) If EXPR is omitted, uses C<$_>.
+
+ print hex '0xAf'; # prints '175'
+ print hex 'aF'; # same
+
+Hex strings may only represent integers. Strings that would cause
+integer overflow trigger a warning. Leading whitespace is not stripped,
+unlike oct(). To present something as hex, look into L</printf>,
+L</sprintf>, and L</unpack>.
+
+=item import LIST
+X<import>
+
+There is no builtin C<import> function. It is just an ordinary
+method (subroutine) defined (or inherited) by modules that wish to export
+names to another module. The C<use> function calls the C<import> method
+for the package used. See also L</use>, L<perlmod>, and L<Exporter>.
+
+=item index STR,SUBSTR,POSITION
+X<index> X<indexOf> X<InStr>
+
+=item index STR,SUBSTR
+
+The index function searches for one string within another, but without
+the wildcard-like behavior of a full regular-expression pattern match.
+It returns the position of the first occurrence of SUBSTR in STR at
+or after POSITION. If POSITION is omitted, starts searching from the
+beginning of the string. POSITION before the beginning of the string
+or after its end is treated as if it were the beginning or the end,
+respectively. POSITION and the return value are based at zero.
+If the substring is not found, C<index> returns -1.
+
+=item int EXPR
+X<int> X<integer> X<truncate> X<trunc> X<floor>
+
+=item int
+
+Returns the integer portion of EXPR. If EXPR is omitted, uses C<$_>.
+You should not use this function for rounding: one because it truncates
+towards C<0>, and two because machine representations of floating-point
+numbers can sometimes produce counterintuitive results. For example,
+C<int(-6.725/0.025)> produces -268 rather than the correct -269; that's
+because it's really more like -268.99999999999994315658 instead. Usually,
+the C<sprintf>, C<printf>, or the C<POSIX::floor> and C<POSIX::ceil>
+functions will serve you better than will int().
+
+=item ioctl FILEHANDLE,FUNCTION,SCALAR
+X<ioctl>
+
+Implements the ioctl(2) function. You'll probably first have to say
+
+ require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
+
+to get the correct function definitions. If F<sys/ioctl.ph> doesn't
+exist or doesn't have the correct definitions you'll have to roll your
+own, based on your C header files such as F<< <sys/ioctl.h> >>.
+(There is a Perl script called B<h2ph> that comes with the Perl kit that
+may help you in this, but it's nontrivial.) SCALAR will be read and/or
+written depending on the FUNCTION; a C pointer to the string value of SCALAR
+will be passed as the third argument of the actual C<ioctl> call. (If SCALAR
+has no string value but does have a numeric value, that value will be
+passed rather than a pointer to the string value. To guarantee this to be
+true, add a C<0> to the scalar before using it.) The C<pack> and C<unpack>
+functions may be needed to manipulate the values of structures used by
+C<ioctl>.
+
+The return value of C<ioctl> (and C<fcntl>) is as follows:
+
+ if OS returns: then Perl returns:
+ -1 undefined value
+ 0 string "0 but true"
+ anything else that number
+
+Thus Perl returns true on success and false on failure, yet you can
+still easily determine the actual value returned by the operating
+system:
+
+ $retval = ioctl(...) || -1;
+ printf "System returned %d\n", $retval;
+
+The special string C<"0 but true"> is exempt from B<-w> complaints
+about improper numeric conversions.
+
+Portability issues: L<perlport/ioctl>.
+
+=item join EXPR,LIST
+X<join>
+
+Joins the separate strings of LIST into a single string with fields
+separated by the value of EXPR, and returns that new string. Example:
+
+ $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
+
+Beware that unlike C<split>, C<join> doesn't take a pattern as its
+first argument. Compare L</split>.
+
+=item keys HASH
+X<keys> X<key>
+
+=item keys ARRAY
+
+=item keys EXPR
+
+Returns a list consisting of all the keys of the named hash, or the indices
+of an array. (In scalar context, returns the number of keys or indices.)
+
+The keys of a hash are returned in an apparently random order. The actual
+random order is subject to change in future versions of Perl, but it
+is guaranteed to be the same order as either the C<values> or C<each>
+function produces (given that the hash has not been modified). Since
+Perl 5.8.1 the ordering can be different even between different runs of
+Perl for security reasons (see L<perlsec/"Algorithmic Complexity
+Attacks">).
+
+As a side effect, calling keys() resets the internal interator of the HASH or ARRAY
+(see L</each>). In particular, calling keys() in void context resets
+the iterator with no other overhead.
+
+Here is yet another way to print your environment:
+
+ @keys = keys %ENV;
+ @values = values %ENV;
+ while (@keys) {
+ print pop(@keys), '=', pop(@values), "\n";
+ }
+
+or how about sorted by key:
+
+ foreach $key (sort(keys %ENV)) {
+ print $key, '=', $ENV{$key}, "\n";
+ }
+
+The returned values are copies of the original keys in the hash, so
+modifying them will not affect the original hash. Compare L</values>.
+
+To sort a hash by value, you'll need to use a C<sort> function.
+Here's a descending numeric sort of a hash by its values:
+
+ foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
+ printf "%4d %s\n", $hash{$key}, $key;
+ }
+
+Used as an lvalue, C<keys> allows you to increase the number of hash buckets
+allocated for the given hash. This can gain you a measure of efficiency if
+you know the hash is going to get big. (This is similar to pre-extending
+an array by assigning a larger number to $#array.) If you say
+
+ keys %hash = 200;
+
+then C<%hash> will have at least 200 buckets allocated for it--256 of them,
+in fact, since it rounds up to the next power of two. These
+buckets will be retained even if you do C<%hash = ()>, use C<undef
+%hash> if you want to free the storage while C<%hash> is still in scope.
+You can't shrink the number of buckets allocated for the hash using
+C<keys> in this way (but you needn't worry about doing this by accident,
+as trying has no effect). C<keys @array> in an lvalue context is a syntax
+error.
+
+Starting with Perl 5.14, C<keys> can take a scalar EXPR, which must contain
+a reference to an unblessed hash or array. The argument will be
+dereferenced automatically. This aspect of C<keys> is considered highly
+experimental. The exact behaviour may change in a future version of Perl.
+
+ for (keys $hashref) { ... }
+ for (keys $obj->get_arrayref) { ... }
+
+See also C<each>, C<values>, and C<sort>.
+
+=item kill SIGNAL, LIST
+X<kill> X<signal>
+
+Sends a signal to a list of processes. Returns the number of
+processes successfully signaled (which is not necessarily the
+same as the number actually killed).
+
+ $cnt = kill 1, $child1, $child2;
+ kill 9, @goners;
+
+If SIGNAL is zero, no signal is sent to the process, but C<kill>
+checks whether it's I<possible> to send a signal to it (that
+means, to be brief, that the process is owned by the same user, or we are
+the super-user). This is useful to check that a child process is still
+alive (even if only as a zombie) and hasn't changed its UID. See
+L<perlport> for notes on the portability of this construct.
+
+Unlike in the shell, if SIGNAL is negative, it kills process groups instead
+of processes. That means you usually want to use positive not negative signals.
+You may also use a signal name in quotes.
+
+The behavior of kill when a I<PROCESS> number is zero or negative depends on
+the operating system. For example, on POSIX-conforming systems, zero will
+signal the current process group and -1 will signal all processes.
+
+See L<perlipc/"Signals"> for more details.
+
+On some platforms such as Windows where the fork() system call is not available.
+Perl can be built to emulate fork() at the interpreter level.
+This emulation has limitations related to kill that have to be considered,
+for code running on Windows and in code intended to be portable.
+
+See L<perlfork> for more details.
+
+Portability issues: L<perlport/kill>.
+
+=item last LABEL
+X<last> X<break>
+
+=item last
+
+The C<last> command is like the C<break> statement in C (as used in
+loops); it immediately exits the loop in question. If the LABEL is
+omitted, the command refers to the innermost enclosing loop. The
+C<continue> block, if any, is not executed:
+
+ LINE: while (<STDIN>) {
+ last LINE if /^$/; # exit when done with header
+ #...
+ }
+
+C<last> cannot be used to exit a block that returns a value such as
+C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
+a grep() or map() operation.
+
+Note that a block by itself is semantically identical to a loop
+that executes once. Thus C<last> can be used to effect an early
+exit out of such a block.
+
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
+=item lc EXPR
+X<lc> X<lowercase>
+
+=item lc
+
+Returns a lowercased version of EXPR. This is the internal function
+implementing the C<\L> escape in double-quoted strings.
+
+If EXPR is omitted, uses C<$_>.
+
+What gets returned depends on several factors:
+
+=over
+
+=item If C<use bytes> is in effect:
+
+=over
+
+=item On EBCDIC platforms
+
+The results are what the C language system call C<tolower()> returns.
+
+=item On ASCII platforms
+
+The results follow ASCII semantics. Only characters C<A-Z> change, to C<a-z>
+respectively.
+
+=back
+
+=item Otherwise, If EXPR has the UTF8 flag set
+
+Unicode semantics are used for the case change.
+
+=item Otherwise, if C<use locale> is in effect
+
+Respects current LC_CTYPE locale. See L<perllocale>.
+
+=item Otherwise, if C<use feature 'unicode_strings'> is in effect:
+
+Unicode semantics are used for the case change.
+
+=item Otherwise:
+
+=over
+
+=item On EBCDIC platforms
+
+The results are what the C language system call C<tolower()> returns.
+
+=item On ASCII platforms
+
+ASCII semantics are used for the case change. The lowercase of any character
+outside the ASCII range is the character itself.
+
+=back
+
+=back
+
+=item lcfirst EXPR
+X<lcfirst> X<lowercase>
+
+=item lcfirst
+
+Returns the value of EXPR with the first character lowercased. This
+is the internal function implementing the C<\l> escape in
+double-quoted strings.
+
+If EXPR is omitted, uses C<$_>.
+
+This function behaves the same way under various pragmata, such as in a locale,
+as L</lc> does.
+
+=item length EXPR
+X<length> X<size>
+
+=item length
+
+Returns the length in I<characters> of the value of EXPR. If EXPR is
+omitted, returns the length of C<$_>. If EXPR is undefined, returns
+C<undef>.
+
+This function cannot be used on an entire array or hash to find out how
+many elements these have. For that, use C<scalar @array> and C<scalar keys
+%hash>, respectively.
+
+Like all Perl character operations, length() normally deals in logical
+characters, not physical bytes. For how many bytes a string encoded as
+UTF-8 would take up, use C<length(Encode::encode_utf8(EXPR))> (you'll have
+to C<use Encode> first). See L<Encode> and L<perlunicode>.
+
+=item __LINE__
+X<__LINE__>
+
+A special token that compiles to the current line number.
+
+=item link OLDFILE,NEWFILE
+X<link>
+
+Creates a new filename linked to the old filename. Returns true for
+success, false otherwise.
+
+Portability issues: L<perlport/link>.
+
+=item listen SOCKET,QUEUESIZE
+X<listen>
+
+Does the same thing that the listen(2) system call does. Returns true if
+it succeeded, false otherwise. See the example in
+L<perlipc/"Sockets: Client/Server Communication">.
+
+=item local EXPR
+X<local>
+
+You really probably want to be using C<my> instead, because C<local> isn't
+what most people think of as "local". See
+L<perlsub/"Private Variables via my()"> for details.
+
+A local modifies the listed variables to be local to the enclosing
+block, file, or eval. If more than one value is listed, the list must
+be placed in parentheses. See L<perlsub/"Temporary Values via local()">
+for details, including issues with tied arrays and hashes.
+
+The C<delete local EXPR> construct can also be used to localize the deletion
+of array/hash elements to the current block.
+See L<perlsub/"Localized deletion of elements of composite types">.
+
+=item localtime EXPR
+X<localtime> X<ctime>
+
+=item localtime
+
+Converts a time as returned by the time function to a 9-element list
+with the time analyzed for the local time zone. Typically used as
+follows:
+
+ # 0 1 2 3 4 5 6 7 8
+ ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
+ localtime(time);
+
+All list elements are numeric and come straight out of the C `struct
+tm'. C<$sec>, C<$min>, and C<$hour> are the seconds, minutes, and hours
+of the specified time.
+
+C<$mday> is the day of the month and C<$mon> the month in
+the range C<0..11>, with 0 indicating January and 11 indicating December.
+This makes it easy to get a month name from a list:
+
+ my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
+ print "$abbr[$mon] $mday";
+ # $mon=9, $mday=18 gives "Oct 18"
+
+C<$year> is the number of years since 1900, B<not> just the last two digits
+of the year. That is, C<$year> is C<123> in year 2023. The proper way
+to get a 4-digit year is simply:
+
+ $year += 1900;
+
+Otherwise you create non-Y2K-compliant programs--and you wouldn't want
+to do that, would you?
+
+To get the last two digits of the year (e.g., "01" in 2001) do:
+
+ $year = sprintf("%02d", $year % 100);
+
+C<$wday> is the day of the week, with 0 indicating Sunday and 3 indicating
+Wednesday. C<$yday> is the day of the year, in the range C<0..364>
+(or C<0..365> in leap years.)
+
+C<$isdst> is true if the specified time occurs during Daylight Saving
+Time, false otherwise.
+
+If EXPR is omitted, C<localtime()> uses the current time (as returned
+by time(3)).
+
+In scalar context, C<localtime()> returns the ctime(3) value:
+
+ $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
+
+This scalar value is B<not> locale-dependent but is a Perl builtin. For GMT
+instead of local time use the L</gmtime> builtin. See also the
+C<Time::Local> module (for converting seconds, minutes, hours, and such back to
+the integer value returned by time()), and the L<POSIX> module's strftime(3)
+and mktime(3) functions.
+
+To get somewhat similar but locale-dependent date strings, set up your
+locale environment variables appropriately (please see L<perllocale>) and
+try for example:
+
+ use POSIX qw(strftime);
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
+ # or for GMT formatted appropriately for your locale:
+ $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
+
+Note that the C<%a> and C<%b>, the short forms of the day of the week
+and the month of the year, may not necessarily be three characters wide.
+
+The L<Time::gmtime> and L<Time::localtime> modules provide a convenient,
+by-name access mechanism to the gmtime() and localtime() functions,
+respectively.
+
+For a comprehensive date and time representation look at the
+L<DateTime> module on CPAN.
+
+Portability issues: L<perlport/localtime>.
+
+=item lock THING
+X<lock>
+
+This function places an advisory lock on a shared variable or referenced
+object contained in I<THING> until the lock goes out of scope.
+
+The value returned is the scalar itself, if the argument is a scalar, or a
+reference, if the argument is a hash, array or subroutine.
+
+lock() is a "weak keyword" : this means that if you've defined a function
+by this name (before any calls to it), that function will be called
+instead. If you are not under C<use threads::shared> this does nothing.
+See L<threads::shared>.
+
+=item log EXPR
+X<log> X<logarithm> X<e> X<ln> X<base>
+
+=item log
+
+Returns the natural logarithm (base I<e>) of EXPR. If EXPR is omitted,
+returns the log of C<$_>. To get the
+log of another base, use basic algebra:
+The base-N log of a number is equal to the natural log of that number
+divided by the natural log of N. For example:
+
+ sub log10 {
+ my $n = shift;
+ return log($n)/log(10);
+ }
+
+See also L</exp> for the inverse operation.
+
+=item lstat FILEHANDLE
+X<lstat>
+
+=item lstat EXPR
+
+=item lstat DIRHANDLE
+
+=item lstat
+
+Does the same thing as the C<stat> function (including setting the
+special C<_> filehandle) but stats a symbolic link instead of the file
+the symbolic link points to. If symbolic links are unimplemented on
+your system, a normal C<stat> is done. For much more detailed
+information, please see the documentation for C<stat>.
+
+If EXPR is omitted, stats C<$_>.
+
+Portability issues: L<perlport/lstat>.
+
+=item m//
+
+The match operator. See L<perlop/"Regexp Quote-Like Operators">.
+
+=item map BLOCK LIST
+X<map>
+
+=item map EXPR,LIST
+
+Evaluates the BLOCK or EXPR for each element of LIST (locally setting
+C<$_> to each element) and returns the list value composed of the
+results of each such evaluation. In scalar context, returns the
+total number of elements so generated. Evaluates BLOCK or EXPR in
+list context, so each element of LIST may produce zero, one, or
+more elements in the returned value.
+
+ @chars = map(chr, @numbers);
+
+translates a list of numbers to the corresponding characters.
+
+ my @squares = map { $_ * $_ } @numbers;
+
+translates a list of numbers to their squared values.
+
+ my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
+
+shows that number of returned elements can differ from the number of
+input elements. To omit an element, return an empty list ().
+This could also be achieved by writing
+
+ my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
+
+which makes the intention more clear.
+
+Map always returns a list, which can be
+assigned to a hash such that the elements
+become key/value pairs. See L<perldata> for more details.
+
+ %hash = map { get_a_key_for($_) => $_ } @array;
+
+is just a funny way to write
+
+ %hash = ();
+ foreach (@array) {
+ $hash{get_a_key_for($_)} = $_;
+ }
+
+Note that C<$_> is an alias to the list value, so it can be used to
+modify the elements of the LIST. While this is useful and supported,
+it can cause bizarre results if the elements of LIST are not variables.
+Using a regular C<foreach> loop for this purpose would be clearer in
+most cases. See also L</grep> for an array composed of those items of
+the original list for which the BLOCK or EXPR evaluates to true.
+
+If C<$_> is lexical in the scope where the C<map> appears (because it has
+been declared with C<my $_>), then, in addition to being locally aliased to
+the list elements, C<$_> keeps being lexical inside the block; that is, it
+can't be seen from the outside, avoiding any potential side-effects.
+
+C<{> starts both hash references and blocks, so C<map { ...> could be either
+the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look
+ahead for the closing C<}> it has to take a guess at which it's dealing with
+based on what it finds just after the C<{>. Usually it gets it right, but if it
+doesn't it won't realize something is wrong until it gets to the C<}> and
+encounters the missing (or unexpected) comma. The syntax error will be
+reported close to the C<}>, but you'll need to change something near the C<{>
+such as using a unary C<+> to give Perl some help:
+
+ %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
+ %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
+ %hash = map { ("\L$_" => 1) } @array # this also works
+ %hash = map { lc($_) => 1 } @array # as does this.
+ %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
+
+ %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
+
+or to force an anon hash constructor use C<+{>:
+
+ @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs comma at end
+
+to get a list of anonymous hashes each with only one entry apiece.
+
+=item mkdir FILENAME,MASK
+X<mkdir> X<md> X<directory, create>
+
+=item mkdir FILENAME
+
+=item mkdir
+
+Creates the directory specified by FILENAME, with permissions
+specified by MASK (as modified by C<umask>). If it succeeds it
+returns true; otherwise it returns false and sets C<$!> (errno).
+MASK defaults to 0777 if omitted, and FILENAME defaults
+to C<$_> if omitted.
+
+In general, it is better to create directories with a permissive MASK
+and let the user modify that with their C<umask> than it is to supply
+a restrictive MASK and give the user no way to be more permissive.
+The exceptions to this rule are when the file or directory should be
+kept private (mail files, for instance). The perlfunc(1) entry on
+C<umask> discusses the choice of MASK in more detail.
+
+Note that according to the POSIX 1003.1-1996 the FILENAME may have any
+number of trailing slashes. Some operating and filesystems do not get
+this right, so Perl automatically removes all trailing slashes to keep
+everyone happy.
+
+To recursively create a directory structure, look at
+the C<mkpath> function of the L<File::Path> module.
+
+=item msgctl ID,CMD,ARG
+X<msgctl>
+
+Calls the System V IPC function msgctl(2). You'll probably have to say
+
+ use IPC::SysV;
+
+first to get the correct constant definitions. If CMD is C<IPC_STAT>,
+then ARG must be a variable that will hold the returned C<msqid_ds>
+structure. Returns like C<ioctl>: the undefined value for error,
+C<"0 but true"> for zero, or the actual return value otherwise. See also
+L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
+C<IPC::Semaphore>.
+
+Portability issues: L<perlport/msgctl>.
+
+=item msgget KEY,FLAGS
+X<msgget>
+
+Calls the System V IPC function msgget(2). Returns the message queue
+id, or C<undef> on error. See also
+L<perlipc/"SysV IPC"> and the documentation for C<IPC::SysV> and
+C<IPC::Msg>.
+
+Portability issues: L<perlport/msgget>.
+
+=item msgrcv ID,VAR,SIZE,TYPE,FLAGS
+X<msgrcv>
+
+Calls the System V IPC function msgrcv to receive a message from
+message queue ID into variable VAR with a maximum message size of
+SIZE. Note that when a message is received, the message type as a
+native long integer will be the first thing in VAR, followed by the
+actual message. This packing may be opened with C<unpack("l! a*")>.
+Taints the variable. Returns true if successful, false
+on error. See also L<perlipc/"SysV IPC"> and the documentation for
+C<IPC::SysV> and C<IPC::SysV::Msg>.
+
+Portability issues: L<perlport/msgrcv>.
+
+=item msgsnd ID,MSG,FLAGS
+X<msgsnd>
+
+Calls the System V IPC function msgsnd to send the message MSG to the
+message queue ID. MSG must begin with the native long integer message
+type, be followed by the length of the actual message, and then finally
+the message itself. This kind of packing can be achieved with
+C<pack("l! a*", $type, $message)>. Returns true if successful,
+false on error. See also the C<IPC::SysV>
+and C<IPC::SysV::Msg> documentation.
+
+Portability issues: L<perlport/msgsnd>.
+
+=item my EXPR
+X<my>
+
+=item my TYPE EXPR
+
+=item my EXPR : ATTRS
+
+=item my TYPE EXPR : ATTRS
+
+A C<my> declares the listed variables to be local (lexically) to the
+enclosing block, file, or C<eval>. If more than one value is listed,
+the list must be placed in parentheses.
+
+The exact semantics and interface of TYPE and ATTRS are still
+evolving. TYPE is currently bound to the use of the C<fields> pragma,
+and attributes are handled using the C<attributes> pragma, or starting
+from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
+L<perlsub/"Private Variables via my()"> for details, and L<fields>,
+L<attributes>, and L<Attribute::Handlers>.
+
+=item next LABEL
+X<next> X<continue>
+
+=item next
+
+The C<next> command is like the C<continue> statement in C; it starts
+the next iteration of the loop:
+
+ LINE: while (<STDIN>) {
+ next LINE if /^#/; # discard comments
+ #...
+ }
+
+Note that if there were a C<continue> block on the above, it would get
+executed even on discarded lines. If LABEL is omitted, the command
+refers to the innermost enclosing loop.
+
+C<next> cannot be used to exit a block which returns a value such as
+C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
+a grep() or map() operation.
+
+Note that a block by itself is semantically identical to a loop
+that executes once. Thus C<next> will exit such a block early.
+
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
+=item no MODULE VERSION LIST
+X<no declarations>
+X<unimporting>
+
+=item no MODULE VERSION
+
+=item no MODULE LIST
+
+=item no MODULE
+
+=item no VERSION
+
+See the C<use> function, of which C<no> is the opposite.
+
+=item oct EXPR
+X<oct> X<octal> X<hex> X<hexadecimal> X<binary> X<bin>
+
+=item oct
+
+Interprets EXPR as an octal string and returns the corresponding
+value. (If EXPR happens to start off with C<0x>, interprets it as a
+hex string. If EXPR starts off with C<0b>, it is interpreted as a
+binary string. Leading whitespace is ignored in all three cases.)
+The following will handle decimal, binary, octal, and hex in standard
+Perl notation:
+
+ $val = oct($val) if $val =~ /^0/;
+
+If EXPR is omitted, uses C<$_>. To go the other way (produce a number
+in octal), use sprintf() or printf():
+
+ $dec_perms = (stat("filename"))[2] & 07777;
+ $oct_perm_str = sprintf "%o", $perms;
+
+The oct() function is commonly used when a string such as C<644> needs
+to be converted into a file mode, for example. Although Perl
+automatically converts strings into numbers as needed, this automatic
+conversion assumes base 10.
+
+Leading white space is ignored without warning, as too are any trailing
+non-digits, such as a decimal point (C<oct> only handles non-negative
+integers, not negative integers or floating point).
+
+=item open FILEHANDLE,EXPR
+X<open> X<pipe> X<file, open> X<fopen>
+
+=item open FILEHANDLE,MODE,EXPR
+
+=item open FILEHANDLE,MODE,EXPR,LIST
+
+=item open FILEHANDLE,MODE,REFERENCE
+
+=item open FILEHANDLE
+
+Opens the file whose filename is given by EXPR, and associates it with
+FILEHANDLE.
+
+Simple examples to open a file for reading:
+
+ open(my $fh, "<", "input.txt")
+ or die "cannot open < input.txt: $!";
+
+and for writing:
+
+ open(my $fh, ">", "output.txt")
+ or die "cannot open > output.txt: $!";
+
+(The following is a comprehensive reference to open(): for a gentler
+introduction you may consider L<perlopentut>.)
+
+If FILEHANDLE is an undefined scalar variable (or array or hash element), a
+new filehandle is autovivified, meaning that the variable is assigned a
+reference to a newly allocated anonymous filehandle. Otherwise if
+FILEHANDLE is an expression, its value is the real filehandle. (This is
+considered a symbolic reference, so C<use strict "refs"> should I<not> be
+in effect.)
+
+If EXPR is omitted, the global (package) scalar variable of the same
+name as the FILEHANDLE contains the filename. (Note that lexical
+variables--those declared with C<my> or C<state>--will not work for this
+purpose; so if you're using C<my> or C<state>, specify EXPR in your
+call to open.)
+
+If three (or more) arguments are specified, the open mode (including
+optional encoding) in the second argument are distinct from the filename in
+the third. If MODE is C<< < >> or nothing, the file is opened for input.
+If MODE is C<< > >>, the file is opened for output, with existing files
+first being truncated ("clobbered") and nonexisting files newly created.
+If MODE is C<<< >> >>>, the file is opened for appending, again being
+created if necessary.
+
+You can put a C<+> in front of the C<< > >> or C<< < >> to
+indicate that you want both read and write access to the file; thus
+C<< +< >> is almost always preferred for read/write updates--the
+C<< +> >> mode would clobber the file first. You cant usually use
+either read-write mode for updating textfiles, since they have
+variable-length records. See the B<-i> switch in L<perlrun> for a
+better approach. The file is created with permissions of C<0666>
+modified by the process's C<umask> value.
+
+These various prefixes correspond to the fopen(3) modes of C<r>,
+C<r+>, C<w>, C<w+>, C<a>, and C<a+>.
+
+In the one- and two-argument forms of the call, the mode and filename
+should be concatenated (in that order), preferably separated by white
+space. You can--but shouldn't--omit the mode in these forms when that mode
+is C<< < >>. It is always safe to use the two-argument form of C<open> if
+the filename argument is a known literal.
+
+For three or more arguments if MODE is C<|->, the filename is
+interpreted as a command to which output is to be piped, and if MODE
+is C<-|>, the filename is interpreted as a command that pipes
+output to us. In the two-argument (and one-argument) form, one should
+replace dash (C<->) with the command.
+See L<perlipc/"Using open() for IPC"> for more examples of this.
+(You are not allowed to C<open> to a command that pipes both in I<and>
+out, but see L<IPC::Open2>, L<IPC::Open3>, and
+L<perlipc/"Bidirectional Communication with Another Process"> for
+alternatives.)
+
+In the form of pipe opens taking three or more arguments, if LIST is specified
+(extra arguments after the command name) then LIST becomes arguments
+to the command invoked if the platform supports it. The meaning of
+C<open> with more than three arguments for non-pipe modes is not yet
+defined, but experimental "layers" may give extra LIST arguments
+meaning.
+
+In the two-argument (and one-argument) form, opening C<< <- >>
+or C<-> opens STDIN and opening C<< >- >> opens STDOUT.
+
+You may (and usually should) use the three-argument form of open to specify
+I/O layers (sometimes referred to as "disciplines") to apply to the handle
+that affect how the input and output are processed (see L<open> and
+L<PerlIO> for more details). For example:
+
+ open(my $fh, "<:encoding(UTF-8)", "filename")
+ || die "can't open UTF-8 encoded filename: $!";
+
+opens the UTF8-encoded file containing Unicode characters;
+see L<perluniintro>. Note that if layers are specified in the
+three-argument form, then default layers stored in ${^OPEN} (see L<perlvar>;
+usually set by the B<open> pragma or the switch B<-CioD>) are ignored.
+Those layers will also be ignored if you specifying a colon with no name
+following it. In that case the default layer for the operating system
+(:raw on Unix, :crlf on Windows) is used.
+
+Open returns nonzero on success, the undefined value otherwise. If
+the C<open> involved a pipe, the return value happens to be the pid of
+the subprocess.
+
+If you're running Perl on a system that distinguishes between text
+files and binary files, then you should check out L</binmode> for tips
+for dealing with this. The key distinction between systems that need
+C<binmode> and those that don't is their text file formats. Systems
+like Unix, Mac OS, and Plan 9, that end lines with a single
+character and encode that character in C as C<"\n"> do not
+need C<binmode>. The rest need it.
+
+When opening a file, it's seldom a good idea to continue
+if the request failed, so C<open> is frequently used with
+C<die>. Even if C<die> won't do what you want (say, in a CGI script,
+where you want to format a suitable error message (but there are
+modules that can help with that problem)) always check
+the return value from opening a file.
+
+As a special case the three-argument form with a read/write mode and the third
+argument being C<undef>:
+
+ open(my $tmp, "+>", undef) or die ...
+
+opens a filehandle to an anonymous temporary file. Also using C<< +< >>
+works for symmetry, but you really should consider writing something
+to the temporary file first. You will need to seek() to do the
+reading.
+
+Since v5.8.0, Perl has built using PerlIO by default. Unless you've
+changed this (such as building Perl with C<Configure -Uuseperlio>), you can
+open filehandles directly to Perl scalars via:
+
+ open($fh, ">", \$variable) || ..
+
+To (re)open C<STDOUT> or C<STDERR> as an in-memory file, close it first:
+
+ close STDOUT;
+ open(STDOUT, ">", \$variable)
+ or die "Can't open STDOUT: $!";
+
+General examples:
+
+ $ARTICLE = 100;
+ open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
+ while (<ARTICLE>) {...
+
+ open(LOG, ">>/usr/spool/news/twitlog"); # (log is reserved)
+ # if the open fails, output is discarded
+
+ open(my $dbase, "+<", "dbase.mine") # open for update
+ or die "Can't open 'dbase.mine' for update: $!";
+
+ open(my $dbase, "+<dbase.mine") # ditto
+ or die "Can't open 'dbase.mine' for update: $!";
+
+ open(ARTICLE, "-|", "caesar <$article") # decrypt article
+ or die "Can't start caesar: $!";
+
+ open(ARTICLE, "caesar <$article |") # ditto
+ or die "Can't start caesar: $!";
+
+ open(EXTRACT, "|sort >Tmp$$") # $$ is our process id
+ or die "Can't start sort: $!";
+
+ # in-memory files
+ open(MEMORY, ">", \$var)
+ or die "Can't open memory file: $!";
+ print MEMORY "foo!\n"; # output will appear in $var
+
+ # process argument list of files along with any includes
+
+ foreach $file (@ARGV) {
+ process($file, "fh00");
+ }
+
+ sub process {
+ my($filename, $input) = @_;
+ $input++; # this is a string increment
+ unless (open($input, "<", $filename)) {
+ print STDERR "Can't open $filename: $!\n";
+ return;
+ }
+
+ local $_;
+ while (<$input>) { # note use of indirection
+ if (/^#include "(.*)"/) {
+ process($1, $input);
+ next;
+ }
+ #... # whatever
+ }
+ }
+
+See L<perliol> for detailed info on PerlIO.
+
+You may also, in the Bourne shell tradition, specify an EXPR beginning
+with C<< >& >>, in which case the rest of the string is interpreted
+as the name of a filehandle (or file descriptor, if numeric) to be
+duped (as C<dup(2)>) and opened. You may use C<&> after C<< > >>,
+C<<< >> >>>, C<< < >>, C<< +> >>, C<<< +>> >>>, and C<< +< >>.
+The mode you specify should match the mode of the original filehandle.
+(Duping a filehandle does not take into account any existing contents
+of IO buffers.) If you use the three-argument form, then you can pass either a
+number, the name of a filehandle, or the normal "reference to a glob".
+
+Here is a script that saves, redirects, and restores C<STDOUT> and
+C<STDERR> using various methods:
+
+ #!/usr/bin/perl
+ open(my $oldout, ">&STDOUT") or die "Can't dup STDOUT: $!";
+ open(OLDERR, ">&", \*STDERR) or die "Can't dup STDERR: $!";
+
+ open(STDOUT, '>', "foo.out") or die "Can't redirect STDOUT: $!";
+ open(STDERR, ">&STDOUT") or die "Can't dup STDOUT: $!";
+
+ select STDERR; $| = 1; # make unbuffered
+ select STDOUT; $| = 1; # make unbuffered
+
+ print STDOUT "stdout 1\n"; # this works for
+ print STDERR "stderr 1\n"; # subprocesses too
+
+ open(STDOUT, ">&", $oldout) or die "Can't dup \$oldout: $!";
+ open(STDERR, ">&OLDERR") or die "Can't dup OLDERR: $!";
+
+ print STDOUT "stdout 2\n";
+ print STDERR "stderr 2\n";
+
+If you specify C<< '<&=X' >>, where C<X> is a file descriptor number
+or a filehandle, then Perl will do an equivalent of C's C<fdopen> of
+that file descriptor (and not call C<dup(2)>); this is more
+parsimonious of file descriptors. For example:
+
+ # open for input, reusing the fileno of $fd
+ open(FILEHANDLE, "<&=$fd")
+
+or
+
+ open(FILEHANDLE, "<&=", $fd)
+
+or
+
+ # open for append, using the fileno of OLDFH
+ open(FH, ">>&=", OLDFH)
+
+or
+
+ open(FH, ">>&=OLDFH")
+
+Being parsimonious on filehandles is also useful (besides being
+parsimonious) for example when something is dependent on file
+descriptors, like for example locking using flock(). If you do just
+C<< open(A, ">>&B") >>, the filehandle A will not have the same file
+descriptor as B, and therefore flock(A) will not flock(B) nor vice
+versa. But with C<< open(A, ">>&=B") >>, the filehandles will share
+the same underlying system file descriptor.
+
+Note that under Perls older than 5.8.0, Perl uses the standard C library's'
+fdopen() to implement the C<=> functionality. On many Unix systems,
+fdopen() fails when file descriptors exceed a certain value, typically 255.
+For Perls 5.8.0 and later, PerlIO is (most often) the default.
+
+You can see whether your Perl was built with PerlIO by running C<perl -V>
+and looking for the C<useperlio=> line. If C<useperlio> is C<define>, you
+have PerlIO; otherwise you don't.
+
+If you open a pipe on the command C<-> (that is, specify either C<|-> or C<-|>
+with the one- or two-argument forms of C<open>),
+an implicit C<fork> is done, so C<open> returns twice: in the parent
+process it returns the pid
+of the child process, and in the child process it returns (a defined) C<0>.
+Use C<defined($pid)> or C<//> to determine whether the open was successful.
+
+For example, use either
+
+ $child_pid = open(FROM_KID, "-|") // die "can't fork: $!";
+
+or
+ $child_pid = open(TO_KID, "|-") // die "can't fork: $!";
+
+followed by
+
+ if ($child_pid) {
+ # am the parent:
+ # either write TO_KID or else read FROM_KID
+ ...
+ wait $child_pid;
+ } else {
+ # am the child; use STDIN/STDOUT normally
+ ...
+ exit;
+ }
+
+The filehandle behaves normally for the parent, but I/O to that
+filehandle is piped from/to the STDOUT/STDIN of the child process.
+In the child process, the filehandle isn't opened--I/O happens from/to
+the new STDOUT/STDIN. Typically this is used like the normal
+piped open when you want to exercise more control over just how the
+pipe command gets executed, such as when running setuid and
+you don't want to have to scan shell commands for metacharacters.
+
+The following blocks are more or less equivalent:
+
+ open(FOO, "|tr '[a-z]' '[A-Z]'");
+ open(FOO, "|-", "tr '[a-z]' '[A-Z]'");
+ open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
+ open(FOO, "|-", "tr", '[a-z]', '[A-Z]');
+
+ open(FOO, "cat -n '$file'|");
+ open(FOO, "-|", "cat -n '$file'");
+ open(FOO, "-|") || exec "cat", "-n", $file;
+ open(FOO, "-|", "cat", "-n", $file);
+
+The last two examples in each block show the pipe as "list form", which is
+not yet supported on all platforms. A good rule of thumb is that if
+your platform has a real C<fork()> (in other words, if your platform is
+Unix, including Linux and MacOS X), you can use the list form. You would
+want to use the list form of the pipe so you can pass literal arguments
+to the command without risk of the shell interpreting any shell metacharacters
+in them. However, this also bars you from opening pipes to commands
+that intentionally contain shell metacharacters, such as:
+
+ open(FOO, "|cat -n | expand -4 | lpr")
+ // die "Can't open pipeline to lpr: $!";
+
+See L<perlipc/"Safe Pipe Opens"> for more examples of this.
+
+Beginning with v5.6.0, Perl will attempt to flush all files opened for
+output before any operation that may do a fork, but this may not be
+supported on some platforms (see L<perlport>). To be safe, you may need
+to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
+of C<IO::Handle> on any open handles.
+
+On systems that support a close-on-exec flag on files, the flag will
+be set for the newly opened file descriptor as determined by the value
+of C<$^F>. See L<perlvar/$^F>.
+
+Closing any piped filehandle causes the parent process to wait for the
+child to finish, then returns the status value in C<$?> and
+C<${^CHILD_ERROR_NATIVE}>.
+
+The filename passed to the one- and two-argument forms of open() will
+have leading and trailing whitespace deleted and normal
+redirection characters honored. This property, known as "magic open",
+can often be used to good effect. A user could specify a filename of
+F<"rsh cat file |">, or you could change certain filenames as needed:
+
+ $filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
+ open(FH, $filename) or die "Can't open $filename: $!";
+
+Use the three-argument form to open a file with arbitrary weird characters in it,
+
+ open(FOO, "<", $file)
+ || die "can't open < $file: $!";
+
+otherwise it's necessary to protect any leading and trailing whitespace:
+
+ $file =~ s#^(\s)#./$1#;
+ open(FOO, "< $file\0")
+ || die "open failed: $!";
+
+(this may not work on some bizarre filesystems). One should
+conscientiously choose between the I<magic> and I<three-argument> form
+of open():
+
+ open(IN, $ARGV[0]) || die "can't open $ARGV[0]: $!";
+
+will allow the user to specify an argument of the form C<"rsh cat file |">,
+but will not work on a filename that happens to have a trailing space, while
+
+ open(IN, "<", $ARGV[0])
+ || die "can't open < $ARGV[0]: $!";
+
+will have exactly the opposite restrictions.
+
+If you want a "real" C C<open> (see L<open(2)> on your system), then you
+should use the C<sysopen> function, which involves no such magic (but may
+use subtly different filemodes than Perl open(), which is mapped to C
+fopen()). This is another way to protect your filenames from
+interpretation. For example:
+
+ use IO::Handle;
+ sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
+ or die "sysopen $path: $!";
+ $oldfh = select(HANDLE); $| = 1; select($oldfh);
+ print HANDLE "stuff $$\n";
+ seek(HANDLE, 0, 0);
+ print "File contains: ", <HANDLE>;
+
+Using the constructor from the C<IO::Handle> package (or one of its
+subclasses, such as C<IO::File> or C<IO::Socket>), you can generate anonymous
+filehandles that have the scope of the variables used to hold them, then
+automatically (but silently) close once their reference counts become
+zero, typically at scope exit:
+
+ use IO::File;
+ #...
+ sub read_myfile_munged {
+ my $ALL = shift;
+ # or just leave it undef to autoviv
+ my $handle = IO::File->new;
+ open($handle, "<", "myfile") or die "myfile: $!";
+ $first = <$handle>
+ or return (); # Automatically closed here.
+ mung($first) or die "mung failed"; # Or here.
+ return (first, <$handle>) if $ALL; # Or here.
+ return $first; # Or here.
+ }
+
+B<WARNING:> The previous example has a bug because the automatic
+close that happens when the refcount on C<handle> does not
+properly detect and report failures. I<Always> close the handle
+yourself and inspect the return value.
+
+ close($handle)
+ || warn "close failed: $!";
+
+See L</seek> for some details about mixing reading and writing.
+
+Portability issues: L<perlport/open>.
+
+=item opendir DIRHANDLE,EXPR
+X<opendir>
+
+Opens a directory named EXPR for processing by C<readdir>, C<telldir>,
+C<seekdir>, C<rewinddir>, and C<closedir>. Returns true if successful.
+DIRHANDLE may be an expression whose value can be used as an indirect
+dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined
+scalar variable (or array or hash element), the variable is assigned a
+reference to a new anonymous dirhandle; that is, it's autovivified.
+DIRHANDLEs have their own namespace separate from FILEHANDLEs.
+
+See the example at C<readdir>.
+
+=item ord EXPR
+X<ord> X<encoding>
+
+=item ord
+
+Returns the numeric value of the first character of EXPR.
+If EXPR is an empty string, returns 0. If EXPR is omitted, uses C<$_>.
+(Note I<character>, not byte.)
+
+For the reverse, see L</chr>.
+See L<perlunicode> for more about Unicode.
+
+=item our EXPR
+X<our> X<global>
+
+=item our TYPE EXPR
+
+=item our EXPR : ATTRS
+
+=item our TYPE EXPR : ATTRS
+
+C<our> associates a simple name with a package variable in the current
+package for use within the current scope. When C<use strict 'vars'> is in
+effect, C<our> lets you use declared global variables without qualifying
+them with package names, within the lexical scope of the C<our> declaration.
+In this way C<our> differs from C<use vars>, which is package-scoped.
+
+Unlike C<my> or C<state>, which allocates storage for a variable and
+associates a simple name with that storage for use within the current
+scope, C<our> associates a simple name with a package (read: global)
+variable in the current package, for use within the current lexical scope.
+In other words, C<our> has the same scoping rules as C<my> or C<state>, but
+does not necessarily create a variable.
+
+If more than one value is listed, the list must be placed
+in parentheses.
+
+ our $foo;
+ our($bar, $baz);
+
+An C<our> declaration declares a global variable that will be visible
+across its entire lexical scope, even across package boundaries. The
+package in which the variable is entered is determined at the point
+of the declaration, not at the point of use. This means the following
+behavior holds:
+
+ package Foo;
+ our $bar; # declares $Foo::bar for rest of lexical scope
+ $bar = 20;
+
+ package Bar;
+ print $bar; # prints 20, as it refers to $Foo::bar
+
+Multiple C<our> declarations with the same name in the same lexical
+scope are allowed if they are in different packages. If they happen
+to be in the same package, Perl will emit warnings if you have asked
+for them, just like multiple C<my> declarations. Unlike a second
+C<my> declaration, which will bind the name to a fresh variable, a
+second C<our> declaration in the same package, in the same scope, is
+merely redundant.
+
+ use warnings;
+ package Foo;
+ our $bar; # declares $Foo::bar for rest of lexical scope
+ $bar = 20;
+
+ package Bar;
+ our $bar = 30; # declares $Bar::bar for rest of lexical scope
+ print $bar; # prints 30
+
+ our $bar; # emits warning but has no other effect
+ print $bar; # still prints 30
+
+An C<our> declaration may also have a list of attributes associated
+with it.
+
+The exact semantics and interface of TYPE and ATTRS are still
+evolving. TYPE is currently bound to the use of C<fields> pragma,
+and attributes are handled using the C<attributes> pragma, or starting
+from Perl 5.8.0 also via the C<Attribute::Handlers> module. See
+L<perlsub/"Private Variables via my()"> for details, and L<fields>,
+L<attributes>, and L<Attribute::Handlers>.
+
+=item pack TEMPLATE,LIST
+X<pack>
+
+Takes a LIST of values and converts it into a string using the rules
+given by the TEMPLATE. The resulting string is the concatenation of
+the converted values. Typically, each converted value looks
+like its machine-level representation. For example, on 32-bit machines
+an integer may be represented by a sequence of 4 bytes, which will in
+Perl be presented as a string that's 4 characters long.
+
+See L<perlpacktut> for an introduction to this function.
+
+The TEMPLATE is a sequence of characters that give the order and type
+of values, as follows:
+
+ a A string with arbitrary binary data, will be null padded.
+ A A text (ASCII) string, will be space padded.
+ Z A null-terminated (ASCIZ) string, will be null padded.
+
+ b A bit string (ascending bit order inside each byte,
+ like vec()).
+ B A bit string (descending bit order inside each byte).
+ h A hex string (low nybble first).
+ H A hex string (high nybble first).
+
+ c A signed char (8-bit) value.
+ C An unsigned char (octet) value.
+ W An unsigned char value (can be greater than 255).
+
+ s A signed short (16-bit) value.
+ S An unsigned short value.
+
+ l A signed long (32-bit) value.
+ L An unsigned long value.
+
+ q A signed quad (64-bit) value.
+ Q An unsigned quad value.
+ (Quads are available only if your system supports 64-bit
+ integer values _and_ if Perl has been compiled to support
+ those. Raises an exception otherwise.)
+
+ i A signed integer value.
+ I A unsigned integer value.
+ (This 'integer' is _at_least_ 32 bits wide. Its exact
+ size depends on what a local C compiler calls 'int'.)
+
+ n An unsigned short (16-bit) in "network" (big-endian) order.
+ N An unsigned long (32-bit) in "network" (big-endian) order.
+ v An unsigned short (16-bit) in "VAX" (little-endian) order.
+ V An unsigned long (32-bit) in "VAX" (little-endian) order.
+
+ j A Perl internal signed integer value (IV).
+ J A Perl internal unsigned integer value (UV).
+
+ f A single-precision float in native format.
+ d A double-precision float in native format.
+
+ F A Perl internal floating-point value (NV) in native format
+ D A float of long-double precision in native format.
+ (Long doubles are available only if your system supports
+ long double values _and_ if Perl has been compiled to
+ support those. Raises an exception otherwise.)
+
+ p A pointer to a null-terminated string.
+ P A pointer to a structure (fixed-length string).
+
+ u A uuencoded string.
+ U A Unicode character number. Encodes to a character in char-
+ acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
+ byte mode.
+
+ w A BER compressed integer (not an ASN.1 BER, see perlpacktut
+ for details). Its bytes represent an unsigned integer in
+ base 128, most significant digit first, with as few digits
+ as possible. Bit eight (the high bit) is set on each byte
+ except the last.
+
+ x A null byte (a.k.a ASCII NUL, "\000", chr(0))
+ X Back up a byte.
+ @ Null-fill or truncate to absolute position, counted from the
+ start of the innermost ()-group.
+ . Null-fill or truncate to absolute position specified by
+ the value.
+ ( Start of a ()-group.
+
+One or more modifiers below may optionally follow certain letters in the
+TEMPLATE (the second column lists letters for which the modifier is valid):
+
+ ! sSlLiI Forces native (short, long, int) sizes instead
+ of fixed (16-/32-bit) sizes.
+
+ xX Make x and X act as alignment commands.
+
+ nNvV Treat integers as signed instead of unsigned.
+
+ @. Specify position as byte offset in the internal
+ representation of the packed string. Efficient but
+ dangerous.
+
+ > sSiIlLqQ Force big-endian byte-order on the type.
+ jJfFdDpP (The "big end" touches the construct.)
+
+ < sSiIlLqQ Force little-endian byte-order on the type.
+ jJfFdDpP (The "little end" touches the construct.)
+
+The C<< > >> and C<< < >> modifiers can also be used on C<()> groups
+to force a particular byte-order on all components in that group,
+including all its subgroups.
+
+The following rules apply:
+
+=over
+
+=item *
+
+Each letter may optionally be followed by a number indicating the repeat
+count. A numeric repeat count may optionally be enclosed in brackets, as
+in C<pack("C[80]", @arr)>. The repeat count gobbles that many values from
+the LIST when used with all format types other than C<a>, C<A>, C<Z>, C<b>,
+C<B>, C<h>, C<H>, C<@>, C<.>, C<x>, C<X>, and C<P>, where it means
+something else, described below. Supplying a C<*> for the repeat count
+instead of a number means to use however many items are left, except for:
+
+=over
+
+=item *
+
+C<@>, C<x>, and C<X>, where it is equivalent to C<0>.
+
+=item *
+
+<.>, where it means relative to the start of the string.
+
+=item *
+
+C<u>, where it is equivalent to 1 (or 45, which here is equivalent).
+
+=back
+
+One can replace a numeric repeat count with a template letter enclosed in
+brackets to use the packed byte length of the bracketed template for the
+repeat count.
+
+For example, the template C<x[L]> skips as many bytes as in a packed long,
+and the template C<"$t X[$t] $t"> unpacks twice whatever $t (when
+variable-expanded) unpacks. If the template in brackets contains alignment
+commands (such as C<x![d]>), its packed length is calculated as if the
+start of the template had the maximal possible alignment.
+
+When used with C<Z>, a C<*> as the repeat count is guaranteed to add a
+trailing null byte, so the resulting string is always one byte longer than
+the byte length of the item itself.
+
+When used with C<@>, the repeat count represents an offset from the start
+of the innermost C<()> group.
+
+When used with C<.>, the repeat count determines the starting position to
+calculate the value offset as follows:
+
+=over
+
+=item *
+
+If the repeat count is C<0>, it's relative to the current position.
+
+=item *
+
+If the repeat count is C<*>, the offset is relative to the start of the
+packed string.
+
+=item *
+
+And if it's an integer I<n>, the offset is relative to the start of the
+I<n>th innermost C<( )> group, or to the start of the string if I<n> is
+bigger then the group level.
+
+=back
+
+The repeat count for C<u> is interpreted as the maximal number of bytes
+to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat
+count should not be more than 65.
+
+=item *
+
+The C<a>, C<A>, and C<Z> types gobble just one value, but pack it as a
+string of length count, padding with nulls or spaces as needed. When
+unpacking, C<A> strips trailing whitespace and nulls, C<Z> strips everything
+after the first null, and C<a> returns data with no stripping at all.
+
+If the value to pack is too long, the result is truncated. If it's too
+long and an explicit count is provided, C<Z> packs only C<$count-1> bytes,
+followed by a null byte. Thus C<Z> always packs a trailing null, except
+when the count is 0.
+
+=item *
+
+Likewise, the C<b> and C<B> formats pack a string that's that many bits long.
+Each such format generates 1 bit of the result. These are typically followed
+by a repeat count like C<B8> or C<B64>.
+
+Each result bit is based on the least-significant bit of the corresponding
+input character, i.e., on C<ord($char)%2>. In particular, characters C<"0">
+and C<"1"> generate bits 0 and 1, as do characters C<"\000"> and C<"\001">.
+
+Starting from the beginning of the input string, each 8-tuple
+of characters is converted to 1 character of output. With format C<b>,
+the first character of the 8-tuple determines the least-significant bit of a
+character; with format C<B>, it determines the most-significant bit of
+a character.
+
+If the length of the input string is not evenly divisible by 8, the
+remainder is packed as if the input string were padded by null characters
+at the end. Similarly during unpacking, "extra" bits are ignored.
+
+If the input string is longer than needed, remaining characters are ignored.
+
+A C<*> for the repeat count uses all characters of the input field.
+On unpacking, bits are converted to a string of C<0>s and C<1>s.
+
+=item *
+
+The C<h> and C<H> formats pack a string that many nybbles (4-bit groups,
+representable as hexadecimal digits, C<"0".."9"> C<"a".."f">) long.
+
+For each such format, pack() generates 4 bits of result.
+With non-alphabetical characters, the result is based on the 4 least-significant
+bits of the input character, i.e., on C<ord($char)%16>. In particular,
+characters C<"0"> and C<"1"> generate nybbles 0 and 1, as do bytes
+C<"\000"> and C<"\001">. For characters C<"a".."f"> and C<"A".."F">, the result
+is compatible with the usual hexadecimal digits, so that C<"a"> and
+C<"A"> both generate the nybble C<0xA==10>. Use only these specific hex
+characters with this format.
+
+Starting from the beginning of the template to pack(), each pair
+of characters is converted to 1 character of output. With format C<h>, the
+first character of the pair determines the least-significant nybble of the
+output character; with format C<H>, it determines the most-significant
+nybble.
+
+If the length of the input string is not even, it behaves as if padded by
+a null character at the end. Similarly, "extra" nybbles are ignored during
+unpacking.
+
+If the input string is longer than needed, extra characters are ignored.
+
+A C<*> for the repeat count uses all characters of the input field. For
+unpack(), nybbles are converted to a string of hexadecimal digits.
+
+=item *
+
+The C<p> format packs a pointer to a null-terminated string. You are
+responsible for ensuring that the string is not a temporary value, as that
+could potentially get deallocated before you got around to using the packed
+result. The C<P> format packs a pointer to a structure of the size indicated
+by the length. A null pointer is created if the corresponding value for
+C<p> or C<P> is C<undef>; similarly with unpack(), where a null pointer
+unpacks into C<undef>.
+
+If your system has a strange pointer size--meaning a pointer is neither as
+big as an int nor as big as a long--it may not be possible to pack or
+unpack pointers in big- or little-endian byte order. Attempting to do
+so raises an exception.
+
+=item *
+
+The C</> template character allows packing and unpacking of a sequence of
+items where the packed structure contains a packed item count followed by
+the packed items themselves. This is useful when the structure you're
+unpacking has encoded the sizes or repeat counts for some of its fields
+within the structure itself as separate fields.
+
+For C<pack>, you write I<length-item>C</>I<sequence-item>, and the
+I<length-item> describes how the length value is packed. Formats likely
+to be of most use are integer-packing ones like C<n> for Java strings,
+C<w> for ASN.1 or SNMP, and C<N> for Sun XDR.
+
+For C<pack>, I<sequence-item> may have a repeat count, in which case
+the minimum of that and the number of available items is used as the argument
+for I<length-item>. If it has no repeat count or uses a '*', the number
+of available items is used.
+
+For C<unpack>, an internal stack of integer arguments unpacked so far is
+used. You write C</>I<sequence-item> and the repeat count is obtained by
+popping off the last element from the stack. The I<sequence-item> must not
+have a repeat count.
+
+If I<sequence-item> refers to a string type (C<"A">, C<"a">, or C<"Z">),
+the I<length-item> is the string length, not the number of strings. With
+an explicit repeat count for pack, the packed string is adjusted to that
+length. For example:
+
+ This code: gives this result:
+
+ unpack("W/a", "\004Gurusamy") ("Guru")
+ unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
+ unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
+
+ pack("n/a* w/a","hello,","world") "\000\006hello,\005world"
+ pack("a/W2", ord("a") .. ord("z")) "2ab"
+
+The I<length-item> is not returned explicitly from C<unpack>.
+
+Supplying a count to the I<length-item> format letter is only useful with
+C<A>, C<a>, or C<Z>. Packing with a I<length-item> of C<a> or C<Z> may
+introduce C<"\000"> characters, which Perl does not regard as legal in
+numeric strings.
+
+=item *
+
+The integer types C<s>, C<S>, C<l>, and C<L> may be
+followed by a C<!> modifier to specify native shorts or
+longs. As shown in the example above, a bare C<l> means
+exactly 32 bits, although the native C<long> as seen by the local C compiler
+may be larger. This is mainly an issue on 64-bit platforms. You can
+see whether using C<!> makes any difference this way:
+
+ printf "format s is %d, s! is %d\n",
+ length pack("s"), length pack("s!");
+
+ printf "format l is %d, l! is %d\n",
+ length pack("l"), length pack("l!");
+
+
+C<i!> and C<I!> are also allowed, but only for completeness' sake:
+they are identical to C<i> and C<I>.
+
+The actual sizes (in bytes) of native shorts, ints, longs, and long
+longs on the platform where Perl was built are also available from
+the command line:
+
+ $ perl -V:{short,int,long{,long}}size
+ shortsize='2';
+ intsize='4';
+ longsize='4';
+ longlongsize='8';
+
+or programmatically via the C<Config> module:
+
+ use Config;
+ print $Config{shortsize}, "\n";
+ print $Config{intsize}, "\n";
+ print $Config{longsize}, "\n";
+ print $Config{longlongsize}, "\n";
+
+C<$Config{longlongsize}> is undefined on systems without
+long long support.
+
+=item *
+
+The integer formats C<s>, C<S>, C<i>, C<I>, C<l>, C<L>, C<j>, and C<J> are
+inherently non-portable between processors and operating systems because
+they obey native byteorder and endianness. For example, a 4-byte integer
+0x12345678 (305419896 decimal) would be ordered natively (arranged in and
+handled by the CPU registers) into bytes as
+
+ 0x12 0x34 0x56 0x78 # big-endian
+ 0x78 0x56 0x34 0x12 # little-endian
+
+Basically, Intel and VAX CPUs are little-endian, while everybody else,
+including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are
+big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used)
+them in little-endian mode, but SGI/Cray uses them in big-endian mode.
+
+The names I<big-endian> and I<little-endian> are comic references to the
+egg-eating habits of the little-endian Lilliputians and the big-endian
+Blefuscudians from the classic Jonathan Swift satire, I<Gulliver's Travels>.
+This entered computer lingo via the paper "On Holy Wars and a Plea for
+Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980.
+
+Some systems may have even weirder byte orders such as
+
+ 0x56 0x78 0x12 0x34
+ 0x34 0x12 0x78 0x56
+
+You can determine your system endianness with this incantation:
+
+ printf("%#02x ", $_) for unpack("W*", pack L=>0x12345678);
+
+The byteorder on the platform where Perl was built is also available
+via L<Config>:
+
+ use Config;
+ print "$Config{byteorder}\n";
+
+or from the command line:
+
+ $ perl -V:byteorder
+
+Byteorders C<"1234"> and C<"12345678"> are little-endian; C<"4321">
+and C<"87654321"> are big-endian.
+
+For portably packed integers, either use the formats C<n>, C<N>, C<v>,
+and C<V> or else use the C<< > >> and C<< < >> modifiers described
+immediately below. See also L<perlport>.
+
+=item *
+
+Starting with Perl 5.9.2, integer and floating-point formats, along with
+the C<p> and C<P> formats and C<()> groups, may all be followed by the
+C<< > >> or C<< < >> endianness modifiers to respectively enforce big-
+or little-endian byte-order. These modifiers are especially useful
+given how C<n>, C<N>, C<v>, and C<V> don't cover signed integers,
+64-bit integers, or floating-point values.
+
+Here are some concerns to keep in mind when using an endianness modifier:
+
+=over
+
+=item *
+
+Exchanging signed integers between different platforms works only
+when all platforms store them in the same format. Most platforms store
+signed integers in two's-complement notation, so usually this is not an issue.
+
+=item *
+
+The C<< > >> or C<< < >> modifiers can only be used on floating-point
+formats on big- or little-endian machines. Otherwise, attempting to
+use them raises an exception.
+
+=item *
+
+Forcing big- or little-endian byte-order on floating-point values for
+data exchange can work only if all platforms use the same
+binary representation such as IEEE floating-point. Even if all
+platforms are using IEEE, there may still be subtle differences. Being able
+to use C<< > >> or C<< < >> on floating-point values can be useful,
+but also dangerous if you don't know exactly what you're doing.
+It is not a general way to portably store floating-point values.
+
+=item *
+
+When using C<< > >> or C<< < >> on a C<()> group, this affects
+all types inside the group that accept byte-order modifiers,
+including all subgroups. It is silently ignored for all other
+types. You are not allowed to override the byte-order within a group
+that already has a byte-order modifier suffix.
+
+=back
+
+=item *
+
+Real numbers (floats and doubles) are in native machine format only.
+Due to the multiplicity of floating-point formats and the lack of a
+standard "network" representation for them, no facility for interchange has been
+made. This means that packed floating-point data written on one machine
+may not be readable on another, even if both use IEEE floating-point
+arithmetic (because the endianness of the memory representation is not part
+of the IEEE spec). See also L<perlport>.
+
+If you know I<exactly> what you're doing, you can use the C<< > >> or C<< < >>
+modifiers to force big- or little-endian byte-order on floating-point values.
+
+Because Perl uses doubles (or long doubles, if configured) internally for
+all numeric calculation, converting from double into float and thence
+to double again loses precision, so C<unpack("f", pack("f", $foo)>)
+will not in general equal $foo.
+
+=item *
+
+Pack and unpack can operate in two modes: character mode (C<C0> mode) where
+the packed string is processed per character, and UTF-8 mode (C<U0> mode)
+where the packed string is processed in its UTF-8-encoded Unicode form on
+a byte-by-byte basis. Character mode is the default unless the format string
+starts with C<U>. You can always switch mode mid-format with an explicit
+C<C0> or C<U0> in the format. This mode remains in effect until the next
+mode change, or until the end of the C<()> group it (directly) applies to.
+
+Using C<C0> to get Unicode characters while using C<U0> to get I<non>-Unicode
+bytes is not necessarily obvious. Probably only the first of these
+is what you want:
+
+ $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
+ perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
+ 03B1.03C9
+ $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
+ perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
+ CE.B1.CF.89
+ $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
+ perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
+ CE.B1.CF.89
+ $ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
+ perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
+ C3.8E.C2.B1.C3.8F.C2.89
+
+Those examples also illustrate that you should not try to use
+C<pack>/C<unpack> as a substitute for the L<Encode> module.
+
+=item *
+
+You must yourself do any alignment or padding by inserting, for example,
+enough C<"x">es while packing. There is no way for pack() and unpack()
+to know where characters are going to or coming from, so they
+handle their output and input as flat sequences of characters.
+
+=item *
+
+A C<()> group is a sub-TEMPLATE enclosed in parentheses. A group may
+take a repeat count either as postfix, or for unpack(), also via the C</>
+template character. Within each repetition of a group, positioning with
+C<@> starts over at 0. Therefore, the result of
+
+ pack("@1A((@2A)@3A)", qw[X Y Z])
+
+is the string C<"\0X\0\0YZ">.
+
+=item *
+
+C<x> and C<X> accept the C<!> modifier to act as alignment commands: they
+jump forward or back to the closest position aligned at a multiple of C<count>
+characters. For example, to pack() or unpack() a C structure like
+
+ struct {
+ char c; /* one signed, 8-bit character */
+ double d;
+ char cc[2];
+ }
+
+one may need to use the template C<c x![d] d c[2]>. This assumes that
+doubles must be aligned to the size of double.
+
+For alignment commands, a C<count> of 0 is equivalent to a C<count> of 1;
+both are no-ops.
+
+=item *
+
+C<n>, C<N>, C<v> and C<V> accept the C<!> modifier to
+represent signed 16-/32-bit integers in big-/little-endian order.
+This is portable only when all platforms sharing packed data use the
+same binary representation for signed integers; for example, when all
+platforms use two's-complement representation.
+
+=item *
+
+Comments can be embedded in a TEMPLATE using C<#> through the end of line.
+White space can separate pack codes from each other, but modifiers and
+repeat counts must follow immediately. Breaking complex templates into
+individual line-by-line components, suitably annotated, can do as much to
+improve legibility and maintainability of pack/unpack formats as C</x> can
+for complicated pattern matches.
+
+=item *
+
+If TEMPLATE requires more arguments than pack() is given, pack()
+assumes additional C<""> arguments. If TEMPLATE requires fewer arguments
+than given, extra arguments are ignored.
+
+=back
+
+Examples:
+
+ $foo = pack("WWWW",65,66,67,68);
+ # foo eq "ABCD"
+ $foo = pack("W4",65,66,67,68);
+ # same thing
+ $foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
+ # same thing with Unicode circled letters.
+ $foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
+ # same thing with Unicode circled letters. You don't get the
+ # UTF-8 bytes because the U at the start of the format caused
+ # a switch to U0-mode, so the UTF-8 bytes get joined into
+ # characters
+ $foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
+ # foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
+ # This is the UTF-8 encoding of the string in the
+ # previous example
+
+ $foo = pack("ccxxcc",65,66,67,68);
+ # foo eq "AB\0\0CD"
+
+ # NOTE: The examples above featuring "W" and "c" are true
+ # only on ASCII and ASCII-derived systems such as ISO Latin 1
+ # and UTF-8. On EBCDIC systems, the first example would be
+ # $foo = pack("WWWW",193,194,195,196);
+
+ $foo = pack("s2",1,2);
+ # "\001\000\002\000" on little-endian
+ # "\000\001\000\002" on big-endian
+
+ $foo = pack("a4","abcd","x","y","z");
+ # "abcd"
+
+ $foo = pack("aaaa","abcd","x","y","z");
+ # "axyz"
+
+ $foo = pack("a14","abcdefg");
+ # "abcdefg\0\0\0\0\0\0\0"
+
+ $foo = pack("i9pl", gmtime);
+ # a real struct tm (on my system anyway)
+
+ $utmp_template = "Z8 Z8 Z16 L";
+ $utmp = pack($utmp_template, @utmp1);
+ # a struct utmp (BSDish)
+
+ @utmp2 = unpack($utmp_template, $utmp);
+ # "@utmp1" eq "@utmp2"
+
+ sub bintodec {
+ unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
+ }
+
+ $foo = pack('sx2l', 12, 34);
+ # short 12, two zero bytes padding, long 34
+ $bar = pack('s@4l', 12, 34);
+ # short 12, zero fill to position 4, long 34
+ # $foo eq $bar
+ $baz = pack('s.l', 12, 4, 34);
+ # short 12, zero fill to position 4, long 34
+
+ $foo = pack('nN', 42, 4711);
+ # pack big-endian 16- and 32-bit unsigned integers
+ $foo = pack('S>L>', 42, 4711);
+ # exactly the same
+ $foo = pack('s<l<', -42, 4711);
+ # pack little-endian 16- and 32-bit signed integers
+ $foo = pack('(sl)<', -42, 4711);
+ # exactly the same
+
+The same template may generally also be used in unpack().
+
+=item package NAMESPACE
+
+=item package NAMESPACE VERSION
+X<package> X<module> X<namespace> X<version>
+
+=item package NAMESPACE BLOCK
+
+=item package NAMESPACE VERSION BLOCK
+X<package> X<module> X<namespace> X<version>
+
+Declares the BLOCK or the rest of the compilation unit as being in the
+given namespace. The scope of the package declaration is either the
+supplied code BLOCK or, in the absence of a BLOCK, from the declaration
+itself through the end of current scope (the enclosing block, file, or
+C<eval>). That is, the forms without a BLOCK are operative through the end
+of the current scope, just like the C<my>, C<state>, and C<our> operators.
+All unqualified dynamic identifiers in this scope will be in the given
+namespace, except where overridden by another C<package> declaration or
+when they're one of the special identifiers that qualify into C<main::>,
+like C<STDOUT>, C<ARGV>, C<ENV>, and the punctuation variables.
+
+A package statement affects dynamic variables only, including those
+you've used C<local> on, but I<not> lexical variables, which are created
+with C<my>, C<state>, or C<our>. Typically it would be the first
+declaration in a file included by C<require> or C<use>. You can switch into a
+package in more than one place, since this only determines which default
+symbol table the compiler uses for the rest of that block. You can refer to
+identifiers in other packages than the current one by prefixing the identifier
+with the package name and a double colon, as in C<$SomePack::var>
+or C<ThatPack::INPUT_HANDLE>. If package name is omitted, the C<main>
+package as assumed. That is, C<$::sail> is equivalent to
+C<$main::sail> (as well as to C<$main'sail>, still seen in ancient
+code, mostly from Perl 4).
+
+If VERSION is provided, C<package> sets the C<$VERSION> variable in the given
+namespace to a L<version> object with the VERSION provided. VERSION must be a
+"strict" style version number as defined by the L<version> module: a positive
+decimal number (integer or decimal-fraction) without exponentiation or else a
+dotted-decimal v-string with a leading 'v' character and at least three
+components. You should set C<$VERSION> only once per package.
+
+See L<perlmod/"Packages"> for more information about packages, modules,
+and classes. See L<perlsub> for other scoping issues.
+
+=item pipe READHANDLE,WRITEHANDLE
+X<pipe>
+
+Opens a pair of connected pipes like the corresponding system call.
+Note that if you set up a loop of piped processes, deadlock can occur
+unless you are very careful. In addition, note that Perl's pipes use
+IO buffering, so you may need to set C<$|> to flush your WRITEHANDLE
+after each command, depending on the application.
+
+See L<IPC::Open2>, L<IPC::Open3>, and
+L<perlipc/"Bidirectional Communication with Another Process">
+for examples of such things.
+
+On systems that support a close-on-exec flag on files, that flag is set
+on all newly opened file descriptors whose C<fileno>s are I<higher> than
+the current value of $^F (by default 2 for C<STDERR>). See L<perlvar/$^F>.
+
+=item __PACKAGE__
+X<__PACKAGE__>
+
+A special token that returns the name of the package in which it occurs.
+
+=item pop ARRAY
+X<pop> X<stack>
+
+=item pop EXPR
+
+=item pop
+
+Pops and returns the last value of the array, shortening the array by
+one element.
+
+Returns the undefined value if the array is empty, although this may also
+happen at other times. If ARRAY is omitted, pops the C<@ARGV> array in the
+main program, but the C<@_> array in subroutines, just like C<shift>.
+
+Starting with Perl 5.14, C<pop> can take a scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<pop> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
+=item pos SCALAR
+X<pos> X<match, position>
+
+=item pos
+
+Returns the offset of where the last C<m//g> search left off for the
+variable in question (C<$_> is used when the variable is not
+specified). Note that 0 is a valid match offset. C<undef> indicates
+that the search position is reset (usually due to match failure, but
+can also be because no match has yet been run on the scalar).
+
+C<pos> directly accesses the location used by the regexp engine to
+store the offset, so assigning to C<pos> will change that offset, and
+so will also influence the C<\G> zero-width assertion in regular
+expressions. Both of these effects take place for the next match, so
+you can't affect the position with C<pos> during the current match,
+such as in C<(?{pos() = 5})> or C<s//pos() = 5/e>.
+
+Setting C<pos> also resets the I<matched with zero-length> flag, described
+under L<perlre/"Repeated Patterns Matching a Zero-length Substring">.
+
+Because a failed C<m//gc> match doesn't reset the offset, the return
+from C<pos> won't change either in this case. See L<perlre> and
+L<perlop>.
+
+=item print FILEHANDLE LIST
+X<print>
+
+=item print FILEHANDLE
+
+=item print LIST
+
+=item print
+
+Prints a string or a list of strings. Returns true if successful.
+FILEHANDLE may be a scalar variable containing the name of or a reference
+to the filehandle, thus introducing one level of indirection. (NOTE: If
+FILEHANDLE is a variable and the next token is a term, it may be
+misinterpreted as an operator unless you interpose a C<+> or put
+parentheses around the arguments.) If FILEHANDLE is omitted, prints to the
+last selected (see L</select>) output handle. If LIST is omitted, prints
+C<$_> to the currently selected output handle. To use FILEHANDLE alone to
+print the content of C<$_> to it, you must use a real filehandle like
+C<FH>, not an indirect one like C<$fh>. To set the default output handle
+to something other than STDOUT, use the select operation.
+
+The current value of C<$,> (if any) is printed between each LIST item. The
+current value of C<$\> (if any) is printed after the entire LIST has been
+printed. Because print takes a LIST, anything in the LIST is evaluated in
+list context, including any subroutines whose return lists you pass to
+C<print>. Be careful not to follow the print keyword with a left
+parenthesis unless you want the corresponding right parenthesis to
+terminate the arguments to the print; put parentheses around all arguments
+(or interpose a C<+>, but that doesn't look as good).
+
+If you're storing handles in an array or hash, or in general whenever
+you're using any expression more complex than a bareword handle or a plain,
+unsubscripted scalar variable to retrieve it, you will have to use a block
+returning the filehandle value instead, in which case the LIST may not be
+omitted:
+
+ print { $files[$i] } "stuff\n";
+ print { $OK ? STDOUT : STDERR } "stuff\n";
+
+Printing to a closed pipe or socket will generate a SIGPIPE signal. See
+L<perlipc> for more on signal handling.
+
+=item printf FILEHANDLE FORMAT, LIST
+X<printf>
+
+=item printf FILEHANDLE
+
+=item printf FORMAT, LIST
+
+=item printf
+
+Equivalent to C<print FILEHANDLE sprintf(FORMAT, LIST)>, except that C<$\>
+(the output record separator) is not appended. The first argument of the
+list will be interpreted as the C<printf> format. See
+L<sprintf|/sprintf FORMAT, LIST> for an
+explanation of the format argument. If you omit the LIST, C<$_> is used;
+to use FILEHANDLE without a LIST, you must use a real filehandle like
+C<FH>, not an indirect one like C<$fh>. If C<use locale> is in effect and
+POSIX::setlocale() has been called, the character used for the decimal
+separator in formatted floating-point numbers is affected by the LC_NUMERIC
+locale setting. See L<perllocale> and L<POSIX>.
+
+Don't fall into the trap of using a C<printf> when a simple
+C<print> would do. The C<print> is more efficient and less
+error prone.
+
+=item prototype FUNCTION
+X<prototype>
+
+Returns the prototype of a function as a string (or C<undef> if the
+function has no prototype). FUNCTION is a reference to, or the name of,
+the function whose prototype you want to retrieve.
+
+If FUNCTION is a string starting with C<CORE::>, the rest is taken as a
+name for a Perl builtin. If the builtin is not I<overridable> (such as
+C<qw//>) or if its arguments cannot be adequately expressed by a prototype
+(such as C<system>), prototype() returns C<undef>, because the builtin
+does not really behave like a Perl function. Otherwise, the string
+describing the equivalent prototype is returned.
+
+=item push ARRAY,LIST
+X<push> X<stack>
+
+=item push EXPR,LIST
+
+Treats ARRAY as a stack by appending the values of LIST to the end of
+ARRAY. The length of ARRAY increases by the length of LIST. Has the same
+effect as
+
+ for $value (LIST) {
+ $ARRAY[++$#ARRAY] = $value;
+ }
+
+but is more efficient. Returns the number of elements in the array following
+the completed C<push>.
+
+Starting with Perl 5.14, C<push> can take a scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<push> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
+=item q/STRING/
+
+=item qq/STRING/
+
+=item qx/STRING/
+
+=item qw/STRING/
+
+Generalized quotes. See L<perlop/"Quote-Like Operators">.
+
+=item qr/STRING/
+
+Regexp-like quote. See L<perlop/"Regexp Quote-Like Operators">.
+
+=item quotemeta EXPR
+X<quotemeta> X<metacharacter>
+
+=item quotemeta
+
+Returns the value of EXPR with all non-"word"
+characters backslashed. (That is, all characters not matching
+C</[A-Za-z_0-9]/> will be preceded by a backslash in the
+returned string, regardless of any locale settings.)
+This is the internal function implementing
+the C<\Q> escape in double-quoted strings.
+
+If EXPR is omitted, uses C<$_>.
+
+quotemeta (and C<\Q> ... C<\E>) are useful when interpolating strings into
+regular expressions, because by default an interpolated variable will be
+considered a mini-regular expression. For example:
+
+ my $sentence = 'The quick brown fox jumped over the lazy dog';
+ my $substring = 'quick.*?fox';
+ $sentence =~ s{$substring}{big bad wolf};
+
+Will cause C<$sentence> to become C<'The big bad wolf jumped over...'>.
+
+On the other hand:
+
+ my $sentence = 'The quick brown fox jumped over the lazy dog';
+ my $substring = 'quick.*?fox';
+ $sentence =~ s{\Q$substring\E}{big bad wolf};
+
+Or:
+
+ my $sentence = 'The quick brown fox jumped over the lazy dog';
+ my $substring = 'quick.*?fox';
+ my $quoted_substring = quotemeta($substring);
+ $sentence =~ s{$quoted_substring}{big bad wolf};
+
+Will both leave the sentence as is. Normally, when accepting literal string
+input from the user, quotemeta() or C<\Q> must be used.
+
+In Perl 5.14, all characters whose code points are above 127 are not
+quoted in UTF8-encoded strings, but all are quoted in UTF-8 strings.
+It is planned to change this behavior in 5.16, but the exact rules
+haven't been determined yet.
+
+=item rand EXPR
+X<rand> X<random>
+
+=item rand
+
+Returns a random fractional number greater than or equal to C<0> and less
+than the value of EXPR. (EXPR should be positive.) If EXPR is
+omitted, the value C<1> is used. Currently EXPR with the value C<0> is
+also special-cased as C<1> (this was undocumented before Perl 5.8.0
+and is subject to change in future versions of Perl). Automatically calls
+C<srand> unless C<srand> has already been called. See also C<srand>.
+
+Apply C<int()> to the value returned by C<rand()> if you want random
+integers instead of random fractional numbers. For example,
+
+ int(rand(10))
+
+returns a random integer between C<0> and C<9>, inclusive.
+
+(Note: If your rand function consistently returns numbers that are too
+large or too small, then your version of Perl was probably compiled
+with the wrong number of RANDBITS.)
+
+B<C<rand()> is not cryptographically secure. You should not rely
+on it in security-sensitive situations.> As of this writing, a
+number of third-party CPAN modules offer random number generators
+intended by their authors to be cryptographically secure,
+including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
+and L<Math::TrulyRandom>.
+
+=item read FILEHANDLE,SCALAR,LENGTH,OFFSET
+X<read> X<file, read>
+
+=item read FILEHANDLE,SCALAR,LENGTH
+
+Attempts to read LENGTH I<characters> of data into variable SCALAR
+from the specified FILEHANDLE. Returns the number of characters
+actually read, C<0> at end of file, or undef if there was an error (in
+the latter case C<$!> is also set). SCALAR will be grown or shrunk
+so that the last character actually read is the last character of the
+scalar after the read.
+
+An OFFSET may be specified to place the read data at some place in the
+string other than the beginning. A negative OFFSET specifies
+placement at that many characters counting backwards from the end of
+the string. A positive OFFSET greater than the length of SCALAR
+results in the string being padded to the required size with C<"\0">
+bytes before the result of the read is appended.
+
+The call is implemented in terms of either Perl's or your system's native
+fread(3) library function. To get a true read(2) system call, see
+L<sysread|/sysread FILEHANDLE,SCALAR,LENGTH,OFFSET>.
+
+Note the I<characters>: depending on the status of the filehandle,
+either (8-bit) bytes or characters are read. By default, all
+filehandles operate on bytes, but for example if the filehandle has
+been opened with the C<:utf8> I/O layer (see L</open>, and the C<open>
+pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
+characters, not bytes. Similarly for the C<:encoding> pragma:
+in that case pretty much any characters can be read.
+
+=item readdir DIRHANDLE
+X<readdir>
+
+Returns the next directory entry for a directory opened by C<opendir>.
+If used in list context, returns all the rest of the entries in the
+directory. If there are no more entries, returns the undefined value in
+scalar context and the empty list in list context.
+
+If you're planning to filetest the return values out of a C<readdir>, you'd
+better prepend the directory in question. Otherwise, because we didn't
+C<chdir> there, it would have been testing the wrong file.
+
+ opendir(my $dh, $some_dir) || die "can't opendir $some_dir: $!";
+ @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
+ closedir $dh;
+
+As of Perl 5.11.2 you can use a bare C<readdir> in a C<while> loop,
+which will set C<$_> on every iteration.
+
+ opendir(my $dh, $some_dir) || die;
+ while(readdir $dh) {
+ print "$some_dir/$_\n";
+ }
+ closedir $dh;
+
+=item readline EXPR
+
+=item readline
+X<readline> X<gets> X<fgets>
+
+Reads from the filehandle whose typeglob is contained in EXPR (or from
+C<*ARGV> if EXPR is not provided). In scalar context, each call reads and
+returns the next line until end-of-file is reached, whereupon the
+subsequent call returns C<undef>. In list context, reads until end-of-file
+is reached and returns a list of lines. Note that the notion of "line"
+used here is whatever you may have defined with C<$/> or
+C<$INPUT_RECORD_SEPARATOR>). See L<perlvar/"$/">.
+
+When C<$/> is set to C<undef>, when C<readline> is in scalar
+context (i.e., file slurp mode), and when an empty file is read, it
+returns C<''> the first time, followed by C<undef> subsequently.
+
+This is the internal function implementing the C<< <EXPR> >>
+operator, but you can use it directly. The C<< <EXPR> >>
+operator is discussed in more detail in L<perlop/"I/O Operators">.
+
+ $line = <STDIN>;
+ $line = readline(*STDIN); # same thing
+
+If C<readline> encounters an operating system error, C<$!> will be set
+with the corresponding error message. It can be helpful to check
+C<$!> when you are reading from filehandles you don't trust, such as a
+tty or a socket. The following example uses the operator form of
+C<readline> and dies if the result is not defined.
+
+ while ( ! eof($fh) ) {
+ defined( $_ = <$fh> ) or die "readline failed: $!";
+ ...
+ }
+
+Note that you have can't handle C<readline> errors that way with the
+C<ARGV> filehandle. In that case, you have to open each element of
+C<@ARGV> yourself since C<eof> handles C<ARGV> differently.
+
+ foreach my $arg (@ARGV) {
+ open(my $fh, $arg) or warn "Can't open $arg: $!";
+
+ while ( ! eof($fh) ) {
+ defined( $_ = <$fh> )
+ or die "readline failed for $arg: $!";
+ ...
+ }
+ }
+
+=item readlink EXPR
+X<readlink>
+
+=item readlink
+
+Returns the value of a symbolic link, if symbolic links are
+implemented. If not, raises an exception. If there is a system
+error, returns the undefined value and sets C<$!> (errno). If EXPR is
+omitted, uses C<$_>.
+
+Portability issues: L<perlport/readlink>.
+
+=item readpipe EXPR
+
+=item readpipe
+X<readpipe>
+
+EXPR is executed as a system command.
+The collected standard output of the command is returned.
+In scalar context, it comes back as a single (potentially
+multi-line) string. In list context, returns a list of lines
+(however you've defined lines with C<$/> or C<$INPUT_RECORD_SEPARATOR>).
+This is the internal function implementing the C<qx/EXPR/>
+operator, but you can use it directly. The C<qx/EXPR/>
+operator is discussed in more detail in L<perlop/"I/O Operators">.
+If EXPR is omitted, uses C<$_>.
+
+=item recv SOCKET,SCALAR,LENGTH,FLAGS
+X<recv>
+
+Receives a message on a socket. Attempts to receive LENGTH characters
+of data into variable SCALAR from the specified SOCKET filehandle.
+SCALAR will be grown or shrunk to the length actually read. Takes the
+same flags as the system call of the same name. Returns the address
+of the sender if SOCKET's protocol supports this; returns an empty
+string otherwise. If there's an error, returns the undefined value.
+This call is actually implemented in terms of recvfrom(2) system call.
+See L<perlipc/"UDP: Message Passing"> for examples.
+
+Note the I<characters>: depending on the status of the socket, either
+(8-bit) bytes or characters are received. By default all sockets
+operate on bytes, but for example if the socket has been changed using
+binmode() to operate with the C<:encoding(utf8)> I/O layer (see the
+C<open> pragma, L<open>), the I/O will operate on UTF8-encoded Unicode
+characters, not bytes. Similarly for the C<:encoding> pragma: in that
+case pretty much any characters can be read.
+
+=item redo LABEL
+X<redo>
+
+=item redo
+
+The C<redo> command restarts the loop block without evaluating the
+conditional again. The C<continue> block, if any, is not executed. If
+the LABEL is omitted, the command refers to the innermost enclosing
+loop. Programs that want to lie to themselves about what was just input
+normally use this command:
+
+ # a simpleminded Pascal comment stripper
+ # (warning: assumes no { or } in strings)
+ LINE: while (<STDIN>) {
+ while (s|({.*}.*){.*}|$1 |) {}
+ s|{.*}| |;
+ if (s|{.*| |) {
+ $front = $_;
+ while (<STDIN>) {
+ if (/}/) { # end of comment?
+ s|^|$front\{|;
+ redo LINE;
+ }
+ }
+ }
+ print;
+ }
+
+C<redo> cannot be used to retry a block that returns a value such as
+C<eval {}>, C<sub {}>, or C<do {}>, and should not be used to exit
+a grep() or map() operation.
+
+Note that a block by itself is semantically identical to a loop
+that executes once. Thus C<redo> inside such a block will effectively
+turn it into a looping construct.
+
+See also L</continue> for an illustration of how C<last>, C<next>, and
+C<redo> work.
+
+=item ref EXPR
+X<ref> X<reference>
+
+=item ref
+
+Returns a non-empty string if EXPR is a reference, the empty
+string otherwise. If EXPR
+is not specified, C<$_> will be used. The value returned depends on the
+type of thing the reference is a reference to.
+Builtin types include:
+
+ SCALAR
+ ARRAY
+ HASH
+ CODE
+ REF
+ GLOB
+ LVALUE
+ FORMAT
+ IO
+ VSTRING
+ Regexp
+
+If the referenced object has been blessed into a package, then that package
+name is returned instead. You can think of C<ref> as a C<typeof> operator.
+
+ if (ref($r) eq "HASH") {
+ print "r is a reference to a hash.\n";
+ }
+ unless (ref($r)) {
+ print "r is not a reference at all.\n";
+ }
+
+The return value C<LVALUE> indicates a reference to an lvalue that is not
+a variable. You get this from taking the reference of function calls like
+C<pos()> or C<substr()>. C<VSTRING> is returned if the reference points
+to a L<version string|perldata/"Version Strings">.
+
+The result C<Regexp> indicates that the argument is a regular expression
+resulting from C<qr//>.
+
+See also L<perlref>.
+
+=item rename OLDNAME,NEWNAME
+X<rename> X<move> X<mv> X<ren>
+
+Changes the name of a file; an existing file NEWNAME will be
+clobbered. Returns true for success, false otherwise.
+
+Behavior of this function varies wildly depending on your system
+implementation. For example, it will usually not work across file system
+boundaries, even though the system I<mv> command sometimes compensates
+for this. Other restrictions include whether it works on directories,
+open files, or pre-existing files. Check L<perlport> and either the
+rename(2) manpage or equivalent system documentation for details.
+
+For a platform independent C<move> function look at the L<File::Copy>
+module.
+
+Portability issues: L<perlport/rename>.
+
+=item require VERSION
+X<require>
+
+=item require EXPR
+
+=item require
+
+Demands a version of Perl specified by VERSION, or demands some semantics
+specified by EXPR or by C<$_> if EXPR is not supplied.
+
+VERSION may be either a numeric argument such as 5.006, which will be
+compared to C<$]>, or a literal of the form v5.6.1, which will be compared
+to C<$^V> (aka $PERL_VERSION). An exception is raised if
+VERSION is greater than the version of the current Perl interpreter.
+Compare with L</use>, which can do a similar check at compile time.
+
+Specifying VERSION as a literal of the form v5.6.1 should generally be
+avoided, because it leads to misleading error messages under earlier
+versions of Perl that do not support this syntax. The equivalent numeric
+version should be used instead.
+
+ require v5.6.1; # run time version check
+ require 5.6.1; # ditto
+ require 5.006_001; # ditto; preferred for backwards compatibility
+
+Otherwise, C<require> demands that a library file be included if it
+hasn't already been included. The file is included via the do-FILE
+mechanism, which is essentially just a variety of C<eval> with the
+caveat that lexical variables in the invoking script will be invisible
+to the included code. Has semantics similar to the following subroutine:
+
+ sub require {
+ my ($filename) = @_;
+ if (exists $INC{$filename}) {
+ return 1 if $INC{$filename};
+ die "Compilation failed in require";
+ }
+ my ($realfilename,$result);
+ ITER: {
+ foreach $prefix (@INC) {
+ $realfilename = "$prefix/$filename";
+ if (-f $realfilename) {
+ $INC{$filename} = $realfilename;
+ $result = do $realfilename;
+ last ITER;
+ }
+ }
+ die "Can't find $filename in \@INC";
+ }
+ if ($@) {
+ $INC{$filename} = undef;
+ die $@;
+ } elsif (!$result) {
+ delete $INC{$filename};
+ die "$filename did not return true value";
+ } else {
+ return $result;
+ }
+ }
+
+Note that the file will not be included twice under the same specified
+name.
+
+The file must return true as the last statement to indicate
+successful execution of any initialization code, so it's customary to
+end such a file with C<1;> unless you're sure it'll return true
+otherwise. But it's better just to put the C<1;>, in case you add more
+statements.
+
+If EXPR is a bareword, the require assumes a "F<.pm>" extension and
+replaces "F<::>" with "F</>" in the filename for you,
+to make it easy to load standard modules. This form of loading of
+modules does not risk altering your namespace.
+
+In other words, if you try this:
+
+ require Foo::Bar; # a splendid bareword
+
+The require function will actually look for the "F<Foo/Bar.pm>" file in the
+directories specified in the C<@INC> array.
+
+But if you try this:
+
+ $class = 'Foo::Bar';
+ require $class; # $class is not a bareword
+ #or
+ require "Foo::Bar"; # not a bareword because of the ""
+
+The require function will look for the "F<Foo::Bar>" file in the @INC array and
+will complain about not finding "F<Foo::Bar>" there. In this case you can do:
+
+ eval "require $class";
+
+Now that you understand how C<require> looks for files with a
+bareword argument, there is a little extra functionality going on behind
+the scenes. Before C<require> looks for a "F<.pm>" extension, it will
+first look for a similar filename with a "F<.pmc>" extension. If this file
+is found, it will be loaded in place of any file ending in a "F<.pm>"
+extension.
+
+You can also insert hooks into the import facility by putting Perl code
+directly into the @INC array. There are three forms of hooks: subroutine
+references, array references, and blessed objects.
+
+Subroutine references are the simplest case. When the inclusion system
+walks through @INC and encounters a subroutine, this subroutine gets
+called with two parameters, the first a reference to itself, and the
+second the name of the file to be included (e.g., "F<Foo/Bar.pm>"). The
+subroutine should return either nothing or else a list of up to three
+values in the following order:
+
+=over
+
+=item 1
+
+A filehandle, from which the file will be read.
+
+=item 2
+
+A reference to a subroutine. If there is no filehandle (previous item),
+then this subroutine is expected to generate one line of source code per
+call, writing the line into C<$_> and returning 1, then finally at end of
+file returning 0. If there is a filehandle, then the subroutine will be
+called to act as a simple source filter, with the line as read in C<$_>.
+Again, return 1 for each valid line, and 0 after all lines have been
+returned.
+
+=item 3
+
+Optional state for the subroutine. The state is passed in as C<$_[1]>. A
+reference to the subroutine itself is passed in as C<$_[0]>.
+
+=back
+
+If an empty list, C<undef>, or nothing that matches the first 3 values above
+is returned, then C<require> looks at the remaining elements of @INC.
+Note that this filehandle must be a real filehandle (strictly a typeglob
+or reference to a typeglob, whether blessed or unblessed); tied filehandles
+will be ignored and processing will stop there.
+
+If the hook is an array reference, its first element must be a subroutine
+reference. This subroutine is called as above, but the first parameter is
+the array reference. This lets you indirectly pass arguments to
+the subroutine.
+
+In other words, you can write:
+
+ push @INC, \&my_sub;
+ sub my_sub {
+ my ($coderef, $filename) = @_; # $coderef is \&my_sub
+ ...
+ }
+
+or:
+
+ push @INC, [ \&my_sub, $x, $y, ... ];
+ sub my_sub {
+ my ($arrayref, $filename) = @_;
+ # Retrieve $x, $y, ...
+ my @parameters = @$arrayref[1..$#$arrayref];
+ ...
+ }
+
+If the hook is an object, it must provide an INC method that will be
+called as above, the first parameter being the object itself. (Note that
+you must fully qualify the sub's name, as unqualified C<INC> is always forced
+into package C<main>.) Here is a typical code layout:
+
+ # In Foo.pm
+ package Foo;
+ sub new { ... }
+ sub Foo::INC {
+ my ($self, $filename) = @_;
+ ...
+ }
+
+ # In the main program
+ push @INC, Foo->new(...);
+
+These hooks are also permitted to set the %INC entry
+corresponding to the files they have loaded. See L<perlvar/%INC>.
+
+For a yet-more-powerful import facility, see L</use> and L<perlmod>.
+
+=item reset EXPR
+X<reset>
+
+=item reset
+
+Generally used in a C<continue> block at the end of a loop to clear
+variables and reset C<??> searches so that they work again. The
+expression is interpreted as a list of single characters (hyphens
+allowed for ranges). All variables and arrays beginning with one of
+those letters are reset to their pristine state. If the expression is
+omitted, one-match searches (C<?pattern?>) are reset to match again.
+Only resets variables or searches in the current package. Always returns
+1. Examples:
+
+ reset 'X'; # reset all X variables
+ reset 'a-z'; # reset lower case variables
+ reset; # just reset ?one-time? searches
+
+Resetting C<"A-Z"> is not recommended because you'll wipe out your
+C<@ARGV> and C<@INC> arrays and your C<%ENV> hash. Resets only package
+variables; lexical variables are unaffected, but they clean themselves
+up on scope exit anyway, so you'll probably want to use them instead.
+See L</my>.
+
+=item return EXPR
+X<return>
+
+=item return
+
+Returns from a subroutine, C<eval>, or C<do FILE> with the value
+given in EXPR. Evaluation of EXPR may be in list, scalar, or void
+context, depending on how the return value will be used, and the context
+may vary from one execution to the next (see L</wantarray>). If no EXPR
+is given, returns an empty list in list context, the undefined value in
+scalar context, and (of course) nothing at all in void context.
+
+(In the absence of an explicit C<return>, a subroutine, eval,
+or do FILE automatically returns the value of the last expression
+evaluated.)
+
+=item reverse LIST
+X<reverse> X<rev> X<invert>
+
+In list context, returns a list value consisting of the elements
+of LIST in the opposite order. In scalar context, concatenates the
+elements of LIST and returns a string value with all characters
+in the opposite order.
+
+ print join(", ", reverse "world", "Hello"); # Hello, world
+
+ print scalar reverse "dlrow ,", "olleH"; # Hello, world
+
+Used without arguments in scalar context, reverse() reverses C<$_>.
+
+ $_ = "dlrow ,olleH";
+ print reverse; # No output, list context
+ print scalar reverse; # Hello, world
+
+Note that reversing an array to itself (as in C<@a = reverse @a>) will
+preserve non-existent elements whenever possible, i.e., for non magical
+arrays or tied arrays with C<EXISTS> and C<DELETE> methods.
+
+This operator is also handy for inverting a hash, although there are some
+caveats. If a value is duplicated in the original hash, only one of those
+can be represented as a key in the inverted hash. Also, this has to
+unwind one hash and build a whole new one, which may take some time
+on a large hash, such as from a DBM file.
+
+ %by_name = reverse %by_address; # Invert the hash
+
+=item rewinddir DIRHANDLE
+X<rewinddir>
+
+Sets the current position to the beginning of the directory for the
+C<readdir> routine on DIRHANDLE.
+
+Portability issues: L<perlport/rewinddir>.
+
+=item rindex STR,SUBSTR,POSITION
+X<rindex>
+
+=item rindex STR,SUBSTR
+
+Works just like index() except that it returns the position of the I<last>
+occurrence of SUBSTR in STR. If POSITION is specified, returns the
+last occurrence beginning at or before that position.
+
+=item rmdir FILENAME
+X<rmdir> X<rd> X<directory, remove>
+
+=item rmdir
+
+Deletes the directory specified by FILENAME if that directory is
+empty. If it succeeds it returns true; otherwise it returns false and
+sets C<$!> (errno). If FILENAME is omitted, uses C<$_>.
+
+To remove a directory tree recursively (C<rm -rf> on Unix) look at
+the C<rmtree> function of the L<File::Path> module.
+
+=item s///
+
+The substitution operator. See L<perlop/"Regexp Quote-Like Operators">.
+
+=item say FILEHANDLE LIST
+X<say>
+
+=item say FILEHANDLE
+
+=item say LIST
+
+=item say
+
+Just like C<print>, but implicitly appends a newline. C<say LIST> is
+simply an abbreviation for C<{ local $\ = "\n"; print LIST }>. To use
+FILEHANDLE without a LIST to print the contents of C<$_> to it, you must
+use a real filehandle like C<FH>, not an indirect one like C<$fh>.
+
+This keyword is available only when the C<"say"> feature
+is enabled, or when prefixed with C<CORE::>; see
+L<feature>. Alternately, include a C<use v5.10> or later to the current
+scope.
+
+=item scalar EXPR
+X<scalar> X<context>
+
+Forces EXPR to be interpreted in scalar context and returns the value
+of EXPR.
+
+ @counts = ( scalar @a, scalar @b, scalar @c );
+
+There is no equivalent operator to force an expression to
+be interpolated in list context because in practice, this is never
+needed. If you really wanted to do so, however, you could use
+the construction C<@{[ (some expression) ]}>, but usually a simple
+C<(some expression)> suffices.
+
+Because C<scalar> is a unary operator, if you accidentally use a
+parenthesized list for the EXPR, this behaves as a scalar comma expression,
+evaluating all but the last element in void context and returning the final
+element evaluated in scalar context. This is seldom what you want.
+
+The following single statement:
+
+ print uc(scalar(&foo,$bar)),$baz;
+
+is the moral equivalent of these two:
+
+ &foo;
+ print(uc($bar),$baz);
+
+See L<perlop> for more details on unary operators and the comma operator.
+
+=item seek FILEHANDLE,POSITION,WHENCE
+X<seek> X<fseek> X<filehandle, position>
+
+Sets FILEHANDLE's position, just like the C<fseek> call of C<stdio>.
+FILEHANDLE may be an expression whose value gives the name of the
+filehandle. The values for WHENCE are C<0> to set the new position
+I<in bytes> to POSITION; C<1> to set it to the current position plus
+POSITION; and C<2> to set it to EOF plus POSITION, typically
+negative. For WHENCE you may use the constants C<SEEK_SET>,
+C<SEEK_CUR>, and C<SEEK_END> (start of the file, current position, end
+of the file) from the L<Fcntl> module. Returns C<1> on success, false
+otherwise.
+
+Note the I<in bytes>: even if the filehandle has been set to
+operate on characters (for example by using the C<:encoding(utf8)> open
+layer), tell() will return byte offsets, not character offsets
+(because implementing that would render seek() and tell() rather slow).
+
+If you want to position the file for C<sysread> or C<syswrite>, don't use
+C<seek>, because buffering makes its effect on the file's read-write position
+unpredictable and non-portable. Use C<sysseek> instead.
+
+Due to the rules and rigors of ANSI C, on some systems you have to do a
+seek whenever you switch between reading and writing. Amongst other
+things, this may have the effect of calling stdio's clearerr(3).
+A WHENCE of C<1> (C<SEEK_CUR>) is useful for not moving the file position:
+
+ seek(TEST,0,1);
+
+This is also useful for applications emulating C<tail -f>. Once you hit
+EOF on your read and then sleep for a while, you (probably) have to stick in a
+dummy seek() to reset things. The C<seek> doesn't change the position,
+but it I<does> clear the end-of-file condition on the handle, so that the
+next C<< <FILE> >> makes Perl try again to read something. (We hope.)
+
+If that doesn't work (some I/O implementations are particularly
+cantankerous), you might need something like this:
+
+ for (;;) {
+ for ($curpos = tell(FILE); $_ = <FILE>;
+ $curpos = tell(FILE)) {
+ # search for some stuff and put it into files
+ }
+ sleep($for_a_while);
+ seek(FILE, $curpos, 0);
+ }
+
+=item seekdir DIRHANDLE,POS
+X<seekdir>
+
+Sets the current position for the C<readdir> routine on DIRHANDLE. POS
+must be a value returned by C<telldir>. C<seekdir> also has the same caveats
+about possible directory compaction as the corresponding system library
+routine.
+
+=item select FILEHANDLE
+X<select> X<filehandle, default>
+
+=item select
+
+Returns the currently selected filehandle. If FILEHANDLE is supplied,
+sets the new current default filehandle for output. This has two
+effects: first, a C<write> or a C<print> without a filehandle
+default to this FILEHANDLE. Second, references to variables related to
+output will refer to this output channel.
+
+For example, to set the top-of-form format for more than one
+output channel, you might do the following:
+
+ select(REPORT1);
+ $^ = 'report1_top';
+ select(REPORT2);
+ $^ = 'report2_top';
+
+FILEHANDLE may be an expression whose value gives the name of the
+actual filehandle. Thus:
+
+ $oldfh = select(STDERR); $| = 1; select($oldfh);
+
+Some programmers may prefer to think of filehandles as objects with
+methods, preferring to write the last example as:
+
+ use IO::Handle;
+ STDERR->autoflush(1);
+
+Portability issues: L<perlport/select>.
+
+=item select RBITS,WBITS,EBITS,TIMEOUT
+X<select>
+
+This calls the select(2) syscall with the bit masks specified, which
+can be constructed using C<fileno> and C<vec>, along these lines:
+
+ $rin = $win = $ein = '';
+ vec($rin, fileno(STDIN), 1) = 1;
+ vec($win, fileno(STDOUT), 1) = 1;
+ $ein = $rin | $win;
+
+If you want to select on many filehandles, you may wish to write a
+subroutine like this:
+
+ sub fhbits {
+ my @fhlist = @_;
+ my $bits = "";
+ for my $fh (@fhlist) {
+ vec($bits, fileno($fh), 1) = 1;
+ }
+ return $bits;
+ }
+ $rin = fhbits(*STDIN, *TTY, *MYSOCK);
+
+The usual idiom is:
+
+ ($nfound,$timeleft) =
+ select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
+
+or to block until something becomes ready just do this
+
+ $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
+
+Most systems do not bother to return anything useful in $timeleft, so
+calling select() in scalar context just returns $nfound.
+
+Any of the bit masks can also be undef. The timeout, if specified, is
+in seconds, which may be fractional. Note: not all implementations are
+capable of returning the $timeleft. If not, they always return
+$timeleft equal to the supplied $timeout.
+
+You can effect a sleep of 250 milliseconds this way:
+
+ select(undef, undef, undef, 0.25);
+
+Note that whether C<select> gets restarted after signals (say, SIGALRM)
+is implementation-dependent. See also L<perlport> for notes on the
+portability of C<select>.
+
+On error, C<select> behaves just like select(2): it returns
+-1 and sets C<$!>.
+
+On some Unixes, select(2) may report a socket file descriptor as "ready for
+reading" even when no data is available, and thus any subsequent C<read>
+would block. This can be avoided if you always use O_NONBLOCK on the
+socket. See select(2) and fcntl(2) for further details.
+
+The standard C<IO::Select> module provides a user-friendlier interface
+to C<select>, mostly because it does all the bit-mask work for you.
+
+B<WARNING>: One should not attempt to mix buffered I/O (like C<read>
+or <FH>) with C<select>, except as permitted by POSIX, and even
+then only on POSIX systems. You have to use C<sysread> instead.
+
+Portability issues: L<perlport/select>.
+
+=item semctl ID,SEMNUM,CMD,ARG
+X<semctl>
+
+Calls the System V IPC function semctl(2). You'll probably have to say
+
+ use IPC::SysV;
+
+first to get the correct constant definitions. If CMD is IPC_STAT or
+GETALL, then ARG must be a variable that will hold the returned
+semid_ds structure or semaphore value array. Returns like C<ioctl>:
+the undefined value for error, "C<0 but true>" for zero, or the actual
+return value otherwise. The ARG must consist of a vector of native
+short integers, which may be created with C<pack("s!",(0)x$nsem)>.
+See also L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::Semaphore>
+documentation.
+
+Portability issues: L<perlport/semctl>.
+
+=item semget KEY,NSEMS,FLAGS
+X<semget>
+
+Calls the System V IPC function semget(2). Returns the semaphore id, or
+the undefined value on error. See also
+L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
+documentation.
+
+Portability issues: L<perlport/semget>.
+
+=item semop KEY,OPSTRING
+X<semop>
+
+Calls the System V IPC function semop(2) for semaphore operations
+such as signalling and waiting. OPSTRING must be a packed array of
+semop structures. Each semop structure can be generated with
+C<pack("s!3", $semnum, $semop, $semflag)>. The length of OPSTRING
+implies the number of semaphore operations. Returns true if
+successful, false on error. As an example, the
+following code waits on semaphore $semnum of semaphore id $semid:
+
+ $semop = pack("s!3", $semnum, -1, 0);
+ die "Semaphore trouble: $!\n" unless semop($semid, $semop);
+
+To signal the semaphore, replace C<-1> with C<1>. See also
+L<perlipc/"SysV IPC">, C<IPC::SysV>, and C<IPC::SysV::Semaphore>
+documentation.
+
+Portability issues: L<perlport/semop>.
+
+=item send SOCKET,MSG,FLAGS,TO
+X<send>
+
+=item send SOCKET,MSG,FLAGS
+
+Sends a message on a socket. Attempts to send the scalar MSG to the SOCKET
+filehandle. Takes the same flags as the system call of the same name. On
+unconnected sockets, you must specify a destination to I<send to>, in which
+case it does a sendto(2) syscall. Returns the number of characters sent,
+or the undefined value on error. The sendmsg(2) syscall is currently
+unimplemented. See L<perlipc/"UDP: Message Passing"> for examples.
+
+Note the I<characters>: depending on the status of the socket, either
+(8-bit) bytes or characters are sent. By default all sockets operate
+on bytes, but for example if the socket has been changed using
+binmode() to operate with the C<:encoding(utf8)> I/O layer (see
+L</open>, or the C<open> pragma, L<open>), the I/O will operate on UTF-8
+encoded Unicode characters, not bytes. Similarly for the C<:encoding>
+pragma: in that case pretty much any characters can be sent.
+
+=item setpgrp PID,PGRP
+X<setpgrp> X<group>
+
+Sets the current process group for the specified PID, C<0> for the current
+process. Raises an exception when used on a machine that doesn't
+implement POSIX setpgid(2) or BSD setpgrp(2). If the arguments are omitted,
+it defaults to C<0,0>. Note that the BSD 4.2 version of C<setpgrp> does not
+accept any arguments, so only C<setpgrp(0,0)> is portable. See also
+C<POSIX::setsid()>.
+
+Portability issues: L<perlport/setpgrp>.
+
+=item setpriority WHICH,WHO,PRIORITY
+X<setpriority> X<priority> X<nice> X<renice>
+
+Sets the current priority for a process, a process group, or a user.
+(See setpriority(2).) Raises an exception when used on a machine
+that doesn't implement setpriority(2).
+
+Portability issues: L<perlport/setpriority>.
+
+=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
+X<setsockopt>
+
+Sets the socket option requested. Returns C<undef> on error.
+Use integer constants provided by the C<Socket> module for
+LEVEL and OPNAME. Values for LEVEL can also be obtained from
+getprotobyname. OPTVAL might either be a packed string or an integer.
+An integer OPTVAL is shorthand for pack("i", OPTVAL).
+
+An example disabling Nagle's algorithm on a socket:
+
+ use Socket qw(IPPROTO_TCP TCP_NODELAY);
+ setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);
+
+Portability issues: L<perlport/setsockopt>.
+
+=item shift ARRAY
+X<shift>
+
+=item shift EXPR
+
+=item shift
+
+Shifts the first value of the array off and returns it, shortening the
+array by 1 and moving everything down. If there are no elements in the
+array, returns the undefined value. If ARRAY is omitted, shifts the
+C<@_> array within the lexical scope of subroutines and formats, and the
+C<@ARGV> array outside a subroutine and also within the lexical scopes
+established by the C<eval STRING>, C<BEGIN {}>, C<INIT {}>, C<CHECK {}>,
+C<UNITCHECK {}>, and C<END {}> constructs.
+
+Starting with Perl 5.14, C<shift> can take a scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<shift> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
+See also C<unshift>, C<push>, and C<pop>. C<shift> and C<unshift> do the
+same thing to the left end of an array that C<pop> and C<push> do to the
+right end.
+
+=item shmctl ID,CMD,ARG
+X<shmctl>
+
+Calls the System V IPC function shmctl. You'll probably have to say
+
+ use IPC::SysV;
+
+first to get the correct constant definitions. If CMD is C<IPC_STAT>,
+then ARG must be a variable that will hold the returned C<shmid_ds>
+structure. Returns like ioctl: C<undef> for error; "C<0> but
+true" for zero; and the actual return value otherwise.
+See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
+
+Portability issues: L<perlport/shmctl>.
+
+=item shmget KEY,SIZE,FLAGS
+X<shmget>
+
+Calls the System V IPC function shmget. Returns the shared memory
+segment id, or C<undef> on error.
+See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.
+
+Portability issues: L<perlport/shmget>.
+
+=item shmread ID,VAR,POS,SIZE
+X<shmread>
+X<shmwrite>
+
+=item shmwrite ID,STRING,POS,SIZE
+
+Reads or writes the System V shared memory segment ID starting at
+position POS for size SIZE by attaching to it, copying in/out, and
+detaching from it. When reading, VAR must be a variable that will
+hold the data read. When writing, if STRING is too long, only SIZE
+bytes are used; if STRING is too short, nulls are written to fill out
+SIZE bytes. Return true if successful, false on error.
+shmread() taints the variable. See also L<perlipc/"SysV IPC">,
+C<IPC::SysV>, and the C<IPC::Shareable> module from CPAN.
+
+Portability issues: L<perlport/shmread> and L<perlport/shmwrite>.
+
+=item shutdown SOCKET,HOW
+X<shutdown>
+
+Shuts down a socket connection in the manner indicated by HOW, which
+has the same interpretation as in the syscall of the same name.
+
+ shutdown(SOCKET, 0); # I/we have stopped reading data
+ shutdown(SOCKET, 1); # I/we have stopped writing data
+ shutdown(SOCKET, 2); # I/we have stopped using this socket
+
+This is useful with sockets when you want to tell the other
+side you're done writing but not done reading, or vice versa.
+It's also a more insistent form of close because it also
+disables the file descriptor in any forked copies in other
+processes.
+
+Returns C<1> for success; on error, returns C<undef> if
+the first argument is not a valid filehandle, or returns C<0> and sets
+C<$!> for any other failure.
+
+=item sin EXPR
+X<sin> X<sine> X<asin> X<arcsine>
+
+=item sin
+
+Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
+returns sine of C<$_>.
+
+For the inverse sine operation, you may use the C<Math::Trig::asin>
+function, or use this relation:
+
+ sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
+
+=item sleep EXPR
+X<sleep> X<pause>
+
+=item sleep
+
+Causes the script to sleep for (integer) EXPR seconds, or forever if no
+argument is given. Returns the integer number of seconds actually slept.
+
+May be interrupted if the process receives a signal such as C<SIGALRM>.
+
+ eval {
+ local $SIG{ALARM} = sub { die "Alarm!\n" };
+ sleep;
+ };
+ die $@ unless $@ eq "Alarm!\n";
+
+You probably cannot mix C<alarm> and C<sleep> calls, because C<sleep>
+is often implemented using C<alarm>.
+
+On some older systems, it may sleep up to a full second less than what
+you requested, depending on how it counts seconds. Most modern systems
+always sleep the full amount. They may appear to sleep longer than that,
+however, because your process might not be scheduled right away in a
+busy multitasking system.
+
+For delays of finer granularity than one second, the Time::HiRes module
+(from CPAN, and starting from Perl 5.8 part of the standard
+distribution) provides usleep(). You may also use Perl's four-argument
+version of select() leaving the first three arguments undefined, or you
+might be able to use the C<syscall> interface to access setitimer(2) if
+your system supports it. See L<perlfaq8> for details.
+
+See also the POSIX module's C<pause> function.
+
+=item socket SOCKET,DOMAIN,TYPE,PROTOCOL
+X<socket>
+
+Opens a socket of the specified kind and attaches it to filehandle
+SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for
+the syscall of the same name. You should C<use Socket> first
+to get the proper definitions imported. See the examples in
+L<perlipc/"Sockets: Client/Server Communication">.
+
+On systems that support a close-on-exec flag on files, the flag will
+be set for the newly opened file descriptor, as determined by the
+value of $^F. See L<perlvar/$^F>.
+
+=item socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
+X<socketpair>
+
+Creates an unnamed pair of sockets in the specified domain, of the
+specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as
+for the syscall of the same name. If unimplemented, raises an exception.
+Returns true if successful.
+
+On systems that support a close-on-exec flag on files, the flag will
+be set for the newly opened file descriptors, as determined by the value
+of $^F. See L<perlvar/$^F>.
+
+Some systems defined C<pipe> in terms of C<socketpair>, in which a call
+to C<pipe(Rdr, Wtr)> is essentially:
+
+ use Socket;
+ socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
+ shutdown(Rdr, 1); # no more writing for reader
+ shutdown(Wtr, 0); # no more reading for writer
+
+See L<perlipc> for an example of socketpair use. Perl 5.8 and later will
+emulate socketpair using IP sockets to localhost if your system implements
+sockets but not socketpair.
+
+Portability issues: L<perlport/socketpair>.
+
+=item sort SUBNAME LIST
+X<sort> X<qsort> X<quicksort> X<mergesort>
+
+=item sort BLOCK LIST
+
+=item sort LIST
+
+In list context, this sorts the LIST and returns the sorted list value.
+In scalar context, the behaviour of C<sort()> is undefined.
+
+If SUBNAME or BLOCK is omitted, C<sort>s in standard string comparison
+order. If SUBNAME is specified, it gives the name of a subroutine
+that returns an integer less than, equal to, or greater than C<0>,
+depending on how the elements of the list are to be ordered. (The
+C<< <=> >> and C<cmp> operators are extremely useful in such routines.)
+SUBNAME may be a scalar variable name (unsubscripted), in which case
+the value provides the name of (or a reference to) the actual
+subroutine to use. In place of a SUBNAME, you can provide a BLOCK as
+an anonymous, in-line sort subroutine.
+
+If the subroutine's prototype is C<($$)>, the elements to be compared are
+passed by reference in C<@_>, as for a normal subroutine. This is slower
+than unprototyped subroutines, where the elements to be compared are passed
+into the subroutine as the package global variables $a and $b (see example
+below). Note that in the latter case, it is usually highly counter-productive
+to declare $a and $b as lexicals.
+
+If the subroutine is an XSUB, the elements to be compared are pushed on to
+the stack, the way arguments are usually passed to XSUBs. $a and $b are
+not set.
+
+The values to be compared are always passed by reference and should not
+be modified.
+
+You also cannot exit out of the sort block or subroutine using any of the
+loop control operators described in L<perlsyn> or with C<goto>.
+
+When C<use locale> is in effect, C<sort LIST> sorts LIST according to the
+current collation locale. See L<perllocale>.
+
+sort() returns aliases into the original list, much as a for loop's index
+variable aliases the list elements. That is, modifying an element of a
+list returned by sort() (for example, in a C<foreach>, C<map> or C<grep>)
+actually modifies the element in the original list. This is usually
+something to be avoided when writing clear code.
+
+Perl 5.6 and earlier used a quicksort algorithm to implement sort.
+That algorithm was not stable, so I<could> go quadratic. (A I<stable> sort
+preserves the input order of elements that compare equal. Although
+quicksort's run time is O(NlogN) when averaged over all arrays of
+length N, the time can be O(N**2), I<quadratic> behavior, for some
+inputs.) In 5.7, the quicksort implementation was replaced with
+a stable mergesort algorithm whose worst-case behavior is O(NlogN).
+But benchmarks indicated that for some inputs, on some platforms,
+the original quicksort was faster. 5.8 has a sort pragma for
+limited control of the sort. Its rather blunt control of the
+underlying algorithm may not persist into future Perls, but the
+ability to characterize the input or output in implementation
+independent ways quite probably will. See L<the sort pragma|sort>.
+
+Examples:
+
+ # sort lexically
+ @articles = sort @files;
+
+ # same thing, but with explicit sort routine
+ @articles = sort {$a cmp $b} @files;
+
+ # now case-insensitively
+ @articles = sort {uc($a) cmp uc($b)} @files;
+
+ # same thing in reversed order
+ @articles = sort {$b cmp $a} @files;
+
+ # sort numerically ascending
+ @articles = sort {$a <=> $b} @files;
+
+ # sort numerically descending
+ @articles = sort {$b <=> $a} @files;
+
+ # this sorts the %age hash by value instead of key
+ # using an in-line function
+ @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
+
+ # sort using explicit subroutine name
+ sub byage {
+ $age{$a} <=> $age{$b}; # presuming numeric
+ }
+ @sortedclass = sort byage @class;
+
+ sub backwards { $b cmp $a }
+ @harry = qw(dog cat x Cain Abel);
+ @george = qw(gone chased yz Punished Axed);
+ print sort @harry;
+ # prints AbelCaincatdogx
+ print sort backwards @harry;
+ # prints xdogcatCainAbel
+ print sort @george, 'to', @harry;
+ # prints AbelAxedCainPunishedcatchaseddoggonetoxyz
+
+ # inefficiently sort by descending numeric compare using
+ # the first integer after the first = sign, or the
+ # whole record case-insensitively otherwise
+
+ my @new = sort {
+ ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
+ ||
+ uc($a) cmp uc($b)
+ } @old;
+
+ # same thing, but much more efficiently;
+ # we'll build auxiliary indices instead
+ # for speed
+ my @nums = @caps = ();
+ for (@old) {
+ push @nums, ( /=(\d+)/ ? $1 : undef );
+ push @caps, uc($_);
+ }
+
+ my @new = @old[ sort {
+ $nums[$b] <=> $nums[$a]
+ ||
+ $caps[$a] cmp $caps[$b]
+ } 0..$#old
+ ];
+
+ # same thing, but without any temps
+ @new = map { $_->[0] }
+ sort { $b->[1] <=> $a->[1]
+ ||
+ $a->[2] cmp $b->[2]
+ } map { [$_, /=(\d+)/, uc($_)] } @old;
+
+ # using a prototype allows you to use any comparison subroutine
+ # as a sort subroutine (including other package's subroutines)
+ package other;
+ sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are not set here
+
+ package main;
+ @new = sort other::backwards @old;
+
+ # guarantee stability, regardless of algorithm
+ use sort 'stable';
+ @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
+
+ # force use of mergesort (not portable outside Perl 5.8)
+ use sort '_mergesort'; # note discouraging _
+ @new = sort { substr($a, 3, 5) cmp substr($b, 3, 5) } @old;
+
+Warning: syntactical care is required when sorting the list returned from
+a function. If you want to sort the list returned by the function call
+C<find_records(@key)>, you can use:
+
+ @contact = sort { $a cmp $b } find_records @key;
+ @contact = sort +find_records(@key);
+ @contact = sort &find_records(@key);
+ @contact = sort(find_records(@key));
+
+If instead you want to sort the array @key with the comparison routine
+C<find_records()> then you can use:
+
+ @contact = sort { find_records() } @key;
+ @contact = sort find_records(@key);
+ @contact = sort(find_records @key);
+ @contact = sort(find_records (@key));
+
+If you're using strict, you I<must not> declare $a
+and $b as lexicals. They are package globals. That means
+that if you're in the C<main> package and type
+
+ @articles = sort {$b <=> $a} @files;
+
+then C<$a> and C<$b> are C<$main::a> and C<$main::b> (or C<$::a> and C<$::b>),
+but if you're in the C<FooPack> package, it's the same as typing
+
+ @articles = sort {$FooPack::b <=> $FooPack::a} @files;
+
+The comparison function is required to behave. If it returns
+inconsistent results (sometimes saying C<$x[1]> is less than C<$x[2]> and
+sometimes saying the opposite, for example) the results are not
+well-defined.
+
+Because C<< <=> >> returns C<undef> when either operand is C<NaN>
+(not-a-number), be careful when sorting with a
+comparison function like C<< $a <=> $b >> any lists that might contain a
+C<NaN>. The following example takes advantage that C<NaN != NaN> to
+eliminate any C<NaN>s from the input list.
+
+ @result = sort { $a <=> $b } grep { $_ == $_ } @input;
+
+=item splice ARRAY or EXPR,OFFSET,LENGTH,LIST
+X<splice>
+
+=item splice ARRAY or EXPR,OFFSET,LENGTH
+
+=item splice ARRAY or EXPR,OFFSET
+
+=item splice ARRAY or EXPR
+
+Removes the elements designated by OFFSET and LENGTH from an array, and
+replaces them with the elements of LIST, if any. In list context,
+returns the elements removed from the array. In scalar context,
+returns the last element removed, or C<undef> if no elements are
+removed. The array grows or shrinks as necessary.
+If OFFSET is negative then it starts that far from the end of the array.
+If LENGTH is omitted, removes everything from OFFSET onward.
+If LENGTH is negative, removes the elements from OFFSET onward
+except for -LENGTH elements at the end of the array.
+If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is
+past the end of the array, Perl issues a warning, and splices at the
+end of the array.
+
+The following equivalences hold (assuming C<< $#a >= $i >> )
+
+ push(@a,$x,$y) splice(@a,@a,0,$x,$y)
+ pop(@a) splice(@a,-1)
+ shift(@a) splice(@a,0,1)
+ unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
+ $a[$i] = $y splice(@a,$i,1,$y)
+
+Example, assuming array lengths are passed before arrays:
+
+ sub aeq { # compare two list values
+ my(@a) = splice(@_,0,shift);
+ my(@b) = splice(@_,0,shift);
+ return 0 unless @a == @b; # same len?
+ while (@a) {
+ return 0 if pop(@a) ne pop(@b);
+ }
+ return 1;
+ }
+ if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
+
+Starting with Perl 5.14, C<splice> can take scalar EXPR, which must hold a
+reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<splice> is considered highly experimental.
+The exact behaviour may change in a future version of Perl.
+
+=item split /PATTERN/,EXPR,LIMIT
+X<split>
+
+=item split /PATTERN/,EXPR
+
+=item split /PATTERN/
+
+=item split
+
+Splits the string EXPR into a list of strings and returns that list. By
+default, empty leading fields are preserved, and empty trailing ones are
+deleted. (If all fields are empty, they are considered to be trailing.)
+
+In scalar context, returns the number of fields found.
+
+If EXPR is omitted, splits the C<$_> string. If PATTERN is also omitted,
+splits on whitespace (after skipping any leading whitespace). Anything
+matching PATTERN is taken to be a delimiter separating the fields. (Note
+that the delimiter may be longer than one character.)
+
+If LIMIT is specified and positive, it represents the maximum number
+of fields the EXPR will be split into, though the actual number of
+fields returned depends on the number of times PATTERN matches within
+EXPR. If LIMIT is unspecified or zero, trailing null fields are
+stripped (which potential users of C<pop> would do well to remember).
+If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
+had been specified. Note that splitting an EXPR that evaluates to the
+empty string always returns the empty list, regardless of the LIMIT
+specified.
+
+A pattern matching the empty string (not to be confused with
+an empty pattern C<//>, which is just one member of the set of patterns
+matching the empty string), splits EXPR into individual
+characters. For example:
+
+ print join(':', split(/ */, 'hi there')), "\n";
+
+produces the output 'h:i:t:h:e:r:e'.
+
+As a special case for C<split>, the empty pattern C<//> specifically
+matches the empty string; this is not be confused with the normal use
+of an empty pattern to mean the last successful match. So to split
+a string into individual characters, the following:
+
+ print join(':', split(//, 'hi there')), "\n";
+
+produces the output 'h:i: :t:h:e:r:e'.
+
+Empty leading fields are produced when there are positive-width matches at
+the beginning of the string; a zero-width match at the beginning of
+the string does not produce an empty field. For example:
+
+ print join(':', split(/(?=\w)/, 'hi there!'));
+
+produces the output 'h:i :t:h:e:r:e!'. Empty trailing fields, on the other
+hand, are produced when there is a match at the end of the string (and
+when LIMIT is given and is not 0), regardless of the length of the match.
+For example:
+
+ print join(':', split(//, 'hi there!', -1)), "\n";
+ print join(':', split(/\W/, 'hi there!', -1)), "\n";
+
+produce the output 'h:i: :t:h:e:r:e:!:' and 'hi:there:', respectively,
+both with an empty trailing field.
+
+The LIMIT parameter can be used to split a line partially
+
+ ($login, $passwd, $remainder) = split(/:/, $_, 3);
+
+When assigning to a list, if LIMIT is omitted, or zero, Perl supplies
+a LIMIT one larger than the number of variables in the list, to avoid
+unnecessary work. For the list above LIMIT would have been 4 by
+default. In time critical applications it behooves you not to split
+into more fields than you really need.
+
+If the PATTERN contains parentheses, additional list elements are
+created from each matching substring in the delimiter.
+
+ split(/([,-])/, "1-10,20", 3);
+
+produces the list value
+
+ (1, '-', 10, ',', 20)
+
+If you had the entire header of a normal Unix email message in $header,
+you could split it up into fields and their values this way:
+
+ $header =~ s/\n(?=\s)//g; # fix continuation lines
+ %hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
+
+The pattern C</PATTERN/> may be replaced with an expression to specify
+patterns that vary at runtime. (To do runtime compilation only once,
+use C</$variable/o>.)
+
+As a special case, specifying a PATTERN of space (S<C<' '>>) will split on
+white space just as C<split> with no arguments does. Thus, S<C<split(' ')>> can
+be used to emulate B<awk>'s default behavior, whereas S<C<split(/ /)>>
+will give you as many initial null fields (empty string) as there are leading spaces.
+A C<split> on C</\s+/> is like a S<C<split(' ')>> except that any leading
+whitespace produces a null first field. A C<split> with no arguments
+really does a S<C<split(' ', $_)>> internally.
+
+A PATTERN of C</^/> is treated as if it were C</^/m>, since it isn't
+much use otherwise.
+
+Example:
+
+ open(PASSWD, '/etc/passwd');
+ while (<PASSWD>) {
+ chomp;
+ ($login, $passwd, $uid, $gid,
+ $gcos, $home, $shell) = split(/:/);
+ #...
+ }
+
+As with regular pattern matching, any capturing parentheses that are not
+matched in a C<split()> will be set to C<undef> when returned:
+
+ @fields = split /(A)|B/, "1A2B3";
+ # @fields is (1, 'A', 2, undef, 3)
+
+=item sprintf FORMAT, LIST
+X<sprintf>
+
+Returns a string formatted by the usual C<printf> conventions of the C
+library function C<sprintf>. See below for more details
+and see L<sprintf(3)> or L<printf(3)> on your system for an explanation of
+the general principles.
+
+For example:
+
+ # Format number with up to 8 leading zeroes
+ $result = sprintf("%08d", $number);
+
+ # Round number to 3 digits after decimal point
+ $rounded = sprintf("%.3f", $number);
+
+Perl does its own C<sprintf> formatting: it emulates the C
+function sprintf(3), but doesn't use it except for floating-point
+numbers, and even then only standard modifiers are allowed.
+Non-standard extensions in your local sprintf(3) are
+therefore unavailable from Perl.
+
+Unlike C<printf>, C<sprintf> does not do what you probably mean when you
+pass it an array as your first argument. The array is given scalar context,
+and instead of using the 0th element of the array as the format, Perl will
+use the count of elements in the array as the format, which is almost never
+useful.
+
+Perl's C<sprintf> permits the following universally-known conversions:
+
+ %% a percent sign
+ %c a character with the given number
+ %s a string
+ %d a signed integer, in decimal
+ %u an unsigned integer, in decimal
+ %o an unsigned integer, in octal
+ %x an unsigned integer, in hexadecimal
+ %e a floating-point number, in scientific notation
+ %f a floating-point number, in fixed decimal notation
+ %g a floating-point number, in %e or %f notation
+
+In addition, Perl permits the following widely-supported conversions:
+
+ %X like %x, but using upper-case letters
+ %E like %e, but using an upper-case "E"
+ %G like %g, but with an upper-case "E" (if applicable)
+ %b an unsigned integer, in binary
+ %B like %b, but using an upper-case "B" with the # flag
+ %p a pointer (outputs the Perl value's address in hexadecimal)
+ %n special: *stores* the number of characters output so far
+ into the next variable in the parameter list
+
+Finally, for backward (and we do mean "backward") compatibility, Perl
+permits these unnecessary but widely-supported conversions:
+
+ %i a synonym for %d
+ %D a synonym for %ld
+ %U a synonym for %lu
+ %O a synonym for %lo
+ %F a synonym for %f
+
+Note that the number of exponent digits in the scientific notation produced
+by C<%e>, C<%E>, C<%g> and C<%G> for numbers with the modulus of the
+exponent less than 100 is system-dependent: it may be three or less
+(zero-padded as necessary). In other words, 1.23 times ten to the
+99th may be either "1.23e99" or "1.23e099".
+
+Between the C<%> and the format letter, you may specify several
+additional attributes controlling the interpretation of the format.
+In order, these are:
+
+=over 4
+
+=item format parameter index
+
+An explicit format parameter index, such as C<2$>. By default sprintf
+will format the next unused argument in the list, but this allows you
+to take the arguments out of order:
+
+ printf '%2$d %1$d', 12, 34; # prints "34 12"
+ printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"
+
+=item flags
+
+one or more of:
+
+ space prefix non-negative number with a space
+ + prefix non-negative number with a plus sign
+ - left-justify within the field
+ 0 use zeros, not spaces, to right-justify
+ # ensure the leading "0" for any octal,
+ prefix non-zero hexadecimal with "0x" or "0X",
+ prefix non-zero binary with "0b" or "0B"
+
+For example:
+
+ printf '<% d>', 12; # prints "< 12>"
+ printf '<%+d>', 12; # prints "<+12>"
+ printf '<%6s>', 12; # prints "< 12>"
+ printf '<%-6s>', 12; # prints "<12 >"
+ printf '<%06s>', 12; # prints "<000012>"
+ printf '<%#o>', 12; # prints "<014>"
+ printf '<%#x>', 12; # prints "<0xc>"
+ printf '<%#X>', 12; # prints "<0XC>"
+ printf '<%#b>', 12; # prints "<0b1100>"
+ printf '<%#B>', 12; # prints "<0B1100>"
+
+When a space and a plus sign are given as the flags at once,
+a plus sign is used to prefix a positive number.
+
+ printf '<%+ d>', 12; # prints "<+12>"
+ printf '<% +d>', 12; # prints "<+12>"
+
+When the # flag and a precision are given in the %o conversion,
+the precision is incremented if it's necessary for the leading "0".
+
+ printf '<%#.5o>', 012; # prints "<00012>"
+ printf '<%#.5o>', 012345; # prints "<012345>"
+ printf '<%#.0o>', 0; # prints "<0>"
+
+=item vector flag
+
+This flag tells Perl to interpret the supplied string as a vector of
+integers, one for each character in the string. Perl applies the format to
+each integer in turn, then joins the resulting strings with a separator (a
+dot C<.> by default). This can be useful for displaying ordinal values of
+characters in arbitrary strings:
+
+ printf "%vd", "AB\x{100}"; # prints "65.66.256"
+ printf "version is v%vd\n", $^V; # Perl's version
+
+Put an asterisk C<*> before the C<v> to override the string to
+use to separate the numbers:
+
+ printf "address is %*vX\n", ":", $addr; # IPv6 address
+ printf "bits are %0*v8b\n", " ", $bits; # random bitstring
+
+You can also explicitly specify the argument number to use for
+the join string using something like C<*2$v>; for example:
+
+ printf '%*4$vX %*4$vX %*4$vX', @addr[1..3], ":"; # 3 IPv6 addresses
+
+=item (minimum) width
+
+Arguments are usually formatted to be only as wide as required to
+display the given value. You can override the width by putting
+a number here, or get the width from the next argument (with C<*>)
+or from a specified argument (e.g., with C<*2$>):
+
+ printf "<%s>", "a"; # prints "<a>"
+ printf "<%6s>", "a"; # prints "< a>"
+ printf "<%*s>", 6, "a"; # prints "< a>"
+ printf "<%*2$s>", "a", 6; # prints "< a>"
+ printf "<%2s>", "long"; # prints "<long>" (does not truncate)
+
+If a field width obtained through C<*> is negative, it has the same
+effect as the C<-> flag: left-justification.
+
+=item precision, or maximum width
+X<precision>
+
+You can specify a precision (for numeric conversions) or a maximum
+width (for string conversions) by specifying a C<.> followed by a number.
+For floating-point formats except C<g> and C<G>, this specifies
+how many places right of the decimal point to show (the default being 6).
+For example:
+
+ # these examples are subject to system-specific variation
+ printf '<%f>', 1; # prints "<1.000000>"
+ printf '<%.1f>', 1; # prints "<1.0>"
+ printf '<%.0f>', 1; # prints "<1>"
+ printf '<%e>', 10; # prints "<1.000000e+01>"
+ printf '<%.1e>', 10; # prints "<1.0e+01>"
+
+For "g" and "G", this specifies the maximum number of digits to show,
+including those prior to the decimal point and those after it; for
+example:
+
+ # These examples are subject to system-specific variation.
+ printf '<%g>', 1; # prints "<1>"
+ printf '<%.10g>', 1; # prints "<1>"
+ printf '<%g>', 100; # prints "<100>"
+ printf '<%.1g>', 100; # prints "<1e+02>"
+ printf '<%.2g>', 100.01; # prints "<1e+02>"
+ printf '<%.5g>', 100.01; # prints "<100.01>"
+ printf '<%.4g>', 100.01; # prints "<100>"
+
+For integer conversions, specifying a precision implies that the
+output of the number itself should be zero-padded to this width,
+where the 0 flag is ignored:
+
+ printf '<%.6d>', 1; # prints "<000001>"
+ printf '<%+.6d>', 1; # prints "<+000001>"
+ printf '<%-10.6d>', 1; # prints "<000001 >"
+ printf '<%10.6d>', 1; # prints "< 000001>"
+ printf '<%010.6d>', 1; # prints "< 000001>"
+ printf '<%+10.6d>', 1; # prints "< +000001>"
+
+ printf '<%.6x>', 1; # prints "<000001>"
+ printf '<%#.6x>', 1; # prints "<0x000001>"
+ printf '<%-10.6x>', 1; # prints "<000001 >"
+ printf '<%10.6x>', 1; # prints "< 000001>"
+ printf '<%010.6x>', 1; # prints "< 000001>"
+ printf '<%#10.6x>', 1; # prints "< 0x000001>"
+
+For string conversions, specifying a precision truncates the string
+to fit the specified width:
+
+ printf '<%.5s>', "truncated"; # prints "<trunc>"
+ printf '<%10.5s>', "truncated"; # prints "< trunc>"
+
+You can also get the precision from the next argument using C<.*>:
+
+ printf '<%.6x>', 1; # prints "<000001>"
+ printf '<%.*x>', 6, 1; # prints "<000001>"
+
+If a precision obtained through C<*> is negative, it counts
+as having no precision at all.
+
+ printf '<%.*s>', 7, "string"; # prints "<string>"
+ printf '<%.*s>', 3, "string"; # prints "<str>"
+ printf '<%.*s>', 0, "string"; # prints "<>"
+ printf '<%.*s>', -1, "string"; # prints "<string>"
+
+ printf '<%.*d>', 1, 0; # prints "<0>"
+ printf '<%.*d>', 0, 0; # prints "<>"
+ printf '<%.*d>', -1, 0; # prints "<0>"
+
+You cannot currently get the precision from a specified number,
+but it is intended that this will be possible in the future, for
+example using C<.*2$>:
+
+ printf "<%.*2$x>", 1, 6; # INVALID, but in future will print "<000001>"
+
+=item size
+
+For numeric conversions, you can specify the size to interpret the
+number as using C<l>, C<h>, C<V>, C<q>, C<L>, or C<ll>. For integer
+conversions (C<d u o x X b i D U O>), numbers are usually assumed to be
+whatever the default integer size is on your platform (usually 32 or 64
+bits), but you can override this to use instead one of the standard C types,
+as supported by the compiler used to build Perl:
+
+ hh interpret integer as C type "char" or "unsigned char"
+ on Perl 5.14 or later
+ h interpret integer as C type "short" or "unsigned short"
+ j interpret integer as C type "intmax_t" on Perl 5.14
+ or later, and only with a C99 compiler (unportable)
+ l interpret integer as C type "long" or "unsigned long"
+ q, L, or ll interpret integer as C type "long long", "unsigned long long",
+ or "quad" (typically 64-bit integers)
+ t interpret integer as C type "ptrdiff_t" on Perl 5.14 or later
+ z interpret integer as C type "size_t" on Perl 5.14 or later
+
+As of 5.14, none of these raises an exception if they are not supported on
+your platform. However, if warnings are enabled, a warning of the
+C<printf> warning class is issued on an unsupported conversion flag.
+Should you instead prefer an exception, do this:
+
+ use warnings FATAL => "printf";
+
+If you would like to know about a version dependency before you
+start running the program, put something like this at its top:
+
+ use 5.014; # for hh/j/t/z/ printf modifiers
+
+You can find out whether your Perl supports quads via L<Config>:
+
+ use Config;
+ if ($Config{use64bitint} eq "define" || $Config{longsize} >= 8) {
+ print "Nice quads!\n";
+ }
+
+For floating-point conversions (C<e f g E F G>), numbers are usually assumed
+to be the default floating-point size on your platform (double or long double),
+but you can force "long double" with C<q>, C<L>, or C<ll> if your
+platform supports them. You can find out whether your Perl supports long
+doubles via L<Config>:
+
+ use Config;
+ print "long doubles\n" if $Config{d_longdbl} eq "define";
+
+You can find out whether Perl considers "long double" to be the default
+floating-point size to use on your platform via L<Config>:
+
+ use Config;
+ if ($Config{uselongdouble} eq "define") {
+ print "long doubles by default\n";
+ }
+
+It can also be that long doubles and doubles are the same thing:
+
+ use Config;
+ ($Config{doublesize} == $Config{longdblsize}) &&
+ print "doubles are long doubles\n";
+
+The size specifier C<V> has no effect for Perl code, but is supported for
+compatibility with XS code. It means "use the standard size for a Perl
+integer or floating-point number", which is the default.
+
+=item order of arguments
+
+Normally, sprintf() takes the next unused argument as the value to
+format for each format specification. If the format specification
+uses C<*> to require additional arguments, these are consumed from
+the argument list in the order they appear in the format
+specification I<before> the value to format. Where an argument is
+specified by an explicit index, this does not affect the normal
+order for the arguments, even when the explicitly specified index
+would have been the next argument.
+
+So:
+
+ printf "<%*.*s>", $a, $b, $c;
+
+uses C<$a> for the width, C<$b> for the precision, and C<$c>
+as the value to format; while:
+
+ printf "<%*1$.*s>", $a, $b;
+
+would use C<$a> for the width and precision, and C<$b> as the
+value to format.
+
+Here are some more examples; be aware that when using an explicit
+index, the C<$> may need escaping:
+
+ printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
+ printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
+ printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
+ printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
+
+=back
+
+If C<use locale> is in effect and POSIX::setlocale() has been called,
+the character used for the decimal separator in formatted floating-point
+numbers is affected by the LC_NUMERIC locale. See L<perllocale>
+and L<POSIX>.
+
+=item sqrt EXPR
+X<sqrt> X<root> X<square root>
+
+=item sqrt
+
+Return the positive square root of EXPR. If EXPR is omitted, uses
+C<$_>. Works only for non-negative operands unless you've
+loaded the C<Math::Complex> module.
+
+ use Math::Complex;
+ print sqrt(-4); # prints 2i
+
+=item srand EXPR
+X<srand> X<seed> X<randseed>
+
+=item srand
+
+Sets and returns the random number seed for the C<rand> operator.
+
+The point of the function is to "seed" the C<rand> function so that
+C<rand> can produce a different sequence each time you run your
+program. When called with a parameter, C<srand> uses that for the seed;
+otherwise it (semi-)randomly chooses a seed. In either case, starting with
+Perl 5.14, it returns the seed.
+
+If C<srand()> is not called explicitly, it is called implicitly without a
+parameter at the first use of the C<rand> operator. However, this was not true
+of versions of Perl before 5.004, so if your script will run under older
+Perl versions, it should call C<srand>; otherwise most programs won't call
+C<srand()> at all.
+
+But there are a few situations in recent Perls where programs are likely to
+want to call C<srand>. One is for generating predictable results generally for
+testing or debugging. There, you use C<srand($seed)>, with the same C<$seed>
+each time. Another case is that you may want to call C<srand()>
+after a C<fork()> to avoid child processes sharing the same seed value as the
+parent (and consequently each other).
+
+Do B<not> call C<srand()> (i.e., without an argument) more than once per
+process. The internal state of the random number generator should
+contain more entropy than can be provided by any seed, so calling
+C<srand()> again actually I<loses> randomness.
+
+Most implementations of C<srand> take an integer and will silently
+truncate decimal numbers. This means C<srand(42)> will usually
+produce the same results as C<srand(42.1)>. To be safe, always pass
+C<srand> an integer.
+
+In versions of Perl prior to 5.004 the default seed was just the
+current C<time>. This isn't a particularly good seed, so many old
+programs supply their own seed value (often C<time ^ $$> or C<time ^
+($$ + ($$ << 15))>), but that isn't necessary any more.
+
+Frequently called programs (like CGI scripts) that simply use
+
+ time ^ $$
+
+for a seed can fall prey to the mathematical property that
+
+ a^b == (a+1)^(b+1)
+
+one-third of the time. So don't do that.
+
+A typical use of the returned seed is for a test program which has too many
+combinations to test comprehensively in the time available to it each run. It
+can test a random subset each time, and should there be a failure, log the seed
+used for that run so that it can later be used to reproduce the same results.
+
+B<C<rand()> is not cryptographically secure. You should not rely
+on it in security-sensitive situations.> As of this writing, a
+number of third-party CPAN modules offer random number generators
+intended by their authors to be cryptographically secure,
+including: L<Data::Entropy>, L<Crypt::Random>, L<Math::Random::Secure>,
+and L<Math::TrulyRandom>.
+
+=item stat FILEHANDLE
+X<stat> X<file, status> X<ctime>
+
+=item stat EXPR
+
+=item stat DIRHANDLE
+
+=item stat
+
+Returns a 13-element list giving the status info for a file, either
+the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is
+omitted, it stats C<$_> (not C<_>!). Returns the empty list if C<stat> fails. Typically
+used as follows:
+
+ ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
+ $atime,$mtime,$ctime,$blksize,$blocks)
+ = stat($filename);
+
+Not all fields are supported on all filesystem types. Here are the
+meanings of the fields:
+
+ 0 dev device number of filesystem
+ 1 ino inode number
+ 2 mode file mode (type and permissions)
+ 3 nlink number of (hard) links to the file
+ 4 uid numeric user ID of file's owner
+ 5 gid numeric group ID of file's owner
+ 6 rdev the device identifier (special files only)
+ 7 size total size of file, in bytes
+ 8 atime last access time in seconds since the epoch
+ 9 mtime last modify time in seconds since the epoch
+ 10 ctime inode change time in seconds since the epoch (*)
+ 11 blksize preferred block size for file system I/O
+ 12 blocks actual number of blocks allocated
+
+(The epoch was at 00:00 January 1, 1970 GMT.)
+
+(*) Not all fields are supported on all filesystem types. Notably, the
+ctime field is non-portable. In particular, you cannot expect it to be a
+"creation time"; see L<perlport/"Files and Filesystems"> for details.
+
+If C<stat> is passed the special filehandle consisting of an underline, no
+stat is done, but the current contents of the stat structure from the
+last C<stat>, C<lstat>, or filetest are returned. Example:
+
+ if (-x $file && (($d) = stat(_)) && $d < 0) {
+ print "$file is executable NFS file\n";
+ }
+
+(This works on machines only for which the device number is negative
+under NFS.)
+
+Because the mode contains both the file type and its permissions, you
+should mask off the file type portion and (s)printf using a C<"%o">
+if you want to see the real permissions.
+
+ $mode = (stat($filename))[2];
+ printf "Permissions are %04o\n", $mode & 07777;
+
+In scalar context, C<stat> returns a boolean value indicating success
+or failure, and, if successful, sets the information associated with
+the special filehandle C<_>.
+
+The L<File::stat> module provides a convenient, by-name access mechanism:
+
+ use File::stat;
+ $sb = stat($filename);
+ printf "File is %s, size is %s, perm %04o, mtime %s\n",
+ $filename, $sb->size, $sb->mode & 07777,
+ scalar localtime $sb->mtime;
+
+You can import symbolic mode constants (C<S_IF*>) and functions
+(C<S_IS*>) from the Fcntl module:
+
+ use Fcntl ':mode';
+
+ $mode = (stat($filename))[2];
+
+ $user_rwx = ($mode & S_IRWXU) >> 6;
+ $group_read = ($mode & S_IRGRP) >> 3;
+ $other_execute = $mode & S_IXOTH;
+
+ printf "Permissions are %04o\n", S_IMODE($mode), "\n";
+
+ $is_setuid = $mode & S_ISUID;
+ $is_directory = S_ISDIR($mode);
+
+You could write the last two using the C<-u> and C<-d> operators.
+Commonly available C<S_IF*> constants are:
+
+ # Permissions: read, write, execute, for user, group, others.
+
+ S_IRWXU S_IRUSR S_IWUSR S_IXUSR
+ S_IRWXG S_IRGRP S_IWGRP S_IXGRP
+ S_IRWXO S_IROTH S_IWOTH S_IXOTH
+
+ # Setuid/Setgid/Stickiness/SaveText.
+ # Note that the exact meaning of these is system-dependent.
+
+ S_ISUID S_ISGID S_ISVTX S_ISTXT
+
+ # File types. Not all are necessarily available on
+ # your system.
+
+ S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
+ S_IFIFO S_IFSOCK S_IFWHT S_ENFMT
+
+ # The following are compatibility aliases for S_IRUSR,
+ # S_IWUSR, and S_IXUSR.
+
+ S_IREAD S_IWRITE S_IEXEC
+
+and the C<S_IF*> functions are
+
+ S_IMODE($mode) the part of $mode containing the permission
+ bits and the setuid/setgid/sticky bits
+
+ S_IFMT($mode) the part of $mode containing the file type
+ which can be bit-anded with (for example)
+ S_IFREG or with the following functions
+
+ # The operators -f, -d, -l, -b, -c, -p, and -S.
+
+ S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
+ S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
+
+ # No direct -X operator counterpart, but for the first one
+ # the -g operator is often equivalent. The ENFMT stands for
+ # record flocking enforcement, a platform-dependent feature.
+
+ S_ISENFMT($mode) S_ISWHT($mode)
+
+See your native chmod(2) and stat(2) documentation for more details
+about the C<S_*> constants. To get status info for a symbolic link
+instead of the target file behind the link, use the C<lstat> function.
+
+Portability issues: L<perlport/stat>.
+
+=item state EXPR
+X<state>
+
+=item state TYPE EXPR
+
+=item state EXPR : ATTRS
+
+=item state TYPE EXPR : ATTRS
+
+C<state> declares a lexically scoped variable, just like C<my>.
+However, those variables will never be reinitialized, contrary to
+lexical variables that are reinitialized each time their enclosing block
+is entered.
+See L<perlsub/"Persistent Private Variables"> for details.
+
+C<state> variables are enabled only when the C<use feature "state"> pragma
+is in effect, unless the keyword is written as C<CORE::state>.
+See also L<feature>.
+
+=item study SCALAR
+X<study>
+
+=item study
+
+Takes extra time to study SCALAR (C<$_> if unspecified) in anticipation of
+doing many pattern matches on the string before it is next modified.
+This may or may not save time, depending on the nature and number of
+patterns you are searching and the distribution of character
+frequencies in the string to be searched; you probably want to compare
+run times with and without it to see which is faster. Those loops
+that scan for many short constant strings (including the constant
+parts of more complex patterns) will benefit most.
+(The way C<study> works is this: a linked list of every
+character in the string to be searched is made, so we know, for
+example, where all the C<'k'> characters are. From each search string,
+the rarest character is selected, based on some static frequency tables
+constructed from some C programs and English text. Only those places
+that contain this "rarest" character are examined.)
+
+For example, here is a loop that inserts index producing entries
+before any line containing a certain pattern:
+
+ while (<>) {
+ study;
+ print ".IX foo\n" if /\bfoo\b/;
+ print ".IX bar\n" if /\bbar\b/;
+ print ".IX blurfl\n" if /\bblurfl\b/;
+ # ...
+ print;
+ }
+
+In searching for C</\bfoo\b/>, only locations in C<$_> that contain C<f>
+will be looked at, because C<f> is rarer than C<o>. In general, this is
+a big win except in pathological cases. The only question is whether
+it saves you more time than it took to build the linked list in the
+first place.
+
+Note that if you have to look for strings that you don't know till
+runtime, you can build an entire loop as a string and C<eval> that to
+avoid recompiling all your patterns all the time. Together with
+undefining C<$/> to input entire files as one record, this can be quite
+fast, often faster than specialized programs like fgrep(1). The following
+scans a list of files (C<@files>) for a list of words (C<@words>), and prints
+out the names of those files that contain a match:
+
+ $search = 'while (<>) { study;';
+ foreach $word (@words) {
+ $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
+ }
+ $search .= "}";
+ @ARGV = @files;
+ undef $/;
+ eval $search; # this screams
+ $/ = "\n"; # put back to normal input delimiter
+ foreach $file (sort keys(%seen)) {
+ print $file, "\n";
+ }
+
+=item sub NAME BLOCK
+X<sub>
+
+=item sub NAME (PROTO) BLOCK
+
+=item sub NAME : ATTRS BLOCK
+
+=item sub NAME (PROTO) : ATTRS BLOCK
+
+This is subroutine definition, not a real function I<per se>. Without a
+BLOCK it's just a forward declaration. Without a NAME, it's an anonymous
+function declaration, so does return a value: the CODE ref of the closure
+just created.
+
+See L<perlsub> and L<perlref> for details about subroutines and
+references; see L<attributes> and L<Attribute::Handlers> for more
+information about attributes.
+
+=item __SUB__
+X<__SUB__>
+
+A special token that returns the a reference to the current subroutine, or
+C<undef> outside of a subroutine.
+
+This token is only available under C<use v5.16> or the "current_sub"
+feature. See L<feature>.
+
+=item substr EXPR,OFFSET,LENGTH,REPLACEMENT
+X<substr> X<substring> X<mid> X<left> X<right>
+
+=item substr EXPR,OFFSET,LENGTH
+
+=item substr EXPR,OFFSET
+
+Extracts a substring out of EXPR and returns it. First character is at
+offset zero. If OFFSET is negative, starts
+that far back from the end of the string. If LENGTH is omitted, returns
+everything through the end of the string. If LENGTH is negative, leaves that
+many characters off the end of the string.
+
+ my $s = "The black cat climbed the green tree";
+ my $color = substr $s, 4, 5; # black
+ my $middle = substr $s, 4, -11; # black cat climbed the
+ my $end = substr $s, 14; # climbed the green tree
+ my $tail = substr $s, -4; # tree
+ my $z = substr $s, -4, 2; # tr
+
+You can use the substr() function as an lvalue, in which case EXPR
+must itself be an lvalue. If you assign something shorter than LENGTH,
+the string will shrink, and if you assign something longer than LENGTH,
+the string will grow to accommodate it. To keep the string the same
+length, you may need to pad or chop your value using C<sprintf>.
+
+If OFFSET and LENGTH specify a substring that is partly outside the
+string, only the part within the string is returned. If the substring
+is beyond either end of the string, substr() returns the undefined
+value and produces a warning. When used as an lvalue, specifying a
+substring that is entirely outside the string raises an exception.
+Here's an example showing the behavior for boundary cases:
+
+ my $name = 'fred';
+ substr($name, 4) = 'dy'; # $name is now 'freddy'
+ my $null = substr $name, 6, 2; # returns "" (no warning)
+ my $oops = substr $name, 7; # returns undef, with warning
+ substr($name, 7) = 'gap'; # raises an exception
+
+An alternative to using substr() as an lvalue is to specify the
+replacement string as the 4th argument. This allows you to replace
+parts of the EXPR and return what was there before in one operation,
+just as you can with splice().
+
+ my $s = "The black cat climbed the green tree";
+ my $z = substr $s, 14, 7, "jumped from"; # climbed
+ # $s is now "The black cat jumped from the green tree"
+
+Note that the lvalue returned by the three-argument version of substr() acts as
+a 'magic bullet'; each time it is assigned to, it remembers which part
+of the original string is being modified; for example:
+
+ $x = '1234';
+ for (substr($x,1,2)) {
+ $_ = 'a'; print $x,"\n"; # prints 1a4
+ $_ = 'xyz'; print $x,"\n"; # prints 1xyz4
+ $x = '56789';
+ $_ = 'pq'; print $x,"\n"; # prints 5pq9
+ }
+
+Prior to Perl version 5.9.1, the result of using an lvalue multiple times was
+unspecified.
+
+=item symlink OLDFILE,NEWFILE
+X<symlink> X<link> X<symbolic link> X<link, symbolic>
+
+Creates a new filename symbolically linked to the old filename.
+Returns C<1> for success, C<0> otherwise. On systems that don't support
+symbolic links, raises an exception. To check for that,
+use eval:
+
+ $symlink_exists = eval { symlink("",""); 1 };
+
+Portability issues: L<perlport/symlink>.
+
+=item syscall NUMBER, LIST
+X<syscall> X<system call>
+
+Calls the system call specified as the first element of the list,
+passing the remaining elements as arguments to the system call. If
+unimplemented, raises an exception. The arguments are interpreted
+as follows: if a given argument is numeric, the argument is passed as
+an int. If not, the pointer to the string value is passed. You are
+responsible to make sure a string is pre-extended long enough to
+receive any result that might be written into a string. You can't use a
+string literal (or other read-only string) as an argument to C<syscall>
+because Perl has to assume that any string pointer might be written
+through. If your
+integer arguments are not literals and have never been interpreted in a
+numeric context, you may need to add C<0> to them to force them to look
+like numbers. This emulates the C<syswrite> function (or vice versa):
+
+ require 'syscall.ph'; # may need to run h2ph
+ $s = "hi there\n";
+ syscall(&SYS_write, fileno(STDOUT), $s, length $s);
+
+Note that Perl supports passing of up to only 14 arguments to your syscall,
+which in practice should (usually) suffice.
+
+Syscall returns whatever value returned by the system call it calls.
+If the system call fails, C<syscall> returns C<-1> and sets C<$!> (errno).
+Note that some system calls I<can> legitimately return C<-1>. The proper
+way to handle such calls is to assign C<$!=0> before the call, then
+check the value of C<$!> if C<syscall> returns C<-1>.
+
+There's a problem with C<syscall(&SYS_pipe)>: it returns the file
+number of the read end of the pipe it creates, but there is no way
+to retrieve the file number of the other end. You can avoid this
+problem by using C<pipe> instead.
+
+Portability issues: L<perlport/syscall>.
+
+=item sysopen FILEHANDLE,FILENAME,MODE
+X<sysopen>
+
+=item sysopen FILEHANDLE,FILENAME,MODE,PERMS
+
+Opens the file whose filename is given by FILENAME, and associates it with
+FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real
+filehandle wanted; an undefined scalar will be suitably autovivified. This
+function calls the underlying operating system's I<open>(2) function with the
+parameters FILENAME, MODE, and PERMS.
+
+The possible values and flag bits of the MODE parameter are
+system-dependent; they are available via the standard module C<Fcntl>. See
+the documentation of your operating system's I<open>(2) syscall to see
+which values and flag bits are available. You may combine several flags
+using the C<|>-operator.
+
+Some of the most common values are C<O_RDONLY> for opening the file in
+read-only mode, C<O_WRONLY> for opening the file in write-only mode,
+and C<O_RDWR> for opening the file in read-write mode.
+X<O_RDONLY> X<O_RDWR> X<O_WRONLY>
+
+For historical reasons, some values work on almost every system
+supported by Perl: 0 means read-only, 1 means write-only, and 2
+means read/write. We know that these values do I<not> work under
+OS/390 & VM/ESA Unix and on the Macintosh; you probably don't want to
+use them in new code.
+
+If the file named by FILENAME does not exist and the C<open> call creates
+it (typically because MODE includes the C<O_CREAT> flag), then the value of
+PERMS specifies the permissions of the newly created file. If you omit
+the PERMS argument to C<sysopen>, Perl uses the octal value C<0666>.
+These permission values need to be in octal, and are modified by your
+process's current C<umask>.
+X<O_CREAT>
+
+In many systems the C<O_EXCL> flag is available for opening files in
+exclusive mode. This is B<not> locking: exclusiveness means here that
+if the file already exists, sysopen() fails. C<O_EXCL> may not work
+on network filesystems, and has no effect unless the C<O_CREAT> flag
+is set as well. Setting C<O_CREAT|O_EXCL> prevents the file from
+being opened if it is a symbolic link. It does not protect against
+symbolic links in the file's path.
+X<O_EXCL>
+
+Sometimes you may want to truncate an already-existing file. This
+can be done using the C<O_TRUNC> flag. The behavior of
+C<O_TRUNC> with C<O_RDONLY> is undefined.
+X<O_TRUNC>
+
+You should seldom if ever use C<0644> as argument to C<sysopen>, because
+that takes away the user's option to have a more permissive umask.
+Better to omit it. See the perlfunc(1) entry on C<umask> for more
+on this.
+
+Note that C<sysopen> depends on the fdopen() C library function.
+On many Unix systems, fdopen() is known to fail when file descriptors
+exceed a certain value, typically 255. If you need more file
+descriptors than that, consider rebuilding Perl to use the C<sfio>
+library, or perhaps using the POSIX::open() function.
+
+See L<perlopentut> for a kinder, gentler explanation of opening files.
+
+Portability issues: L<perlport/sysopen>.
+
+=item sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
+X<sysread>
+
+=item sysread FILEHANDLE,SCALAR,LENGTH
+
+Attempts to read LENGTH bytes of data into variable SCALAR from the
+specified FILEHANDLE, using the read(2). It bypasses
+buffered IO, so mixing this with other kinds of reads, C<print>,
+C<write>, C<seek>, C<tell>, or C<eof> can cause confusion because the
+perlio or stdio layers usually buffers data. Returns the number of
+bytes actually read, C<0> at end of file, or undef if there was an
+error (in the latter case C<$!> is also set). SCALAR will be grown or
+shrunk so that the last byte actually read is the last byte of the
+scalar after the read.
+
+An OFFSET may be specified to place the read data at some place in the
+string other than the beginning. A negative OFFSET specifies
+placement at that many characters counting backwards from the end of
+the string. A positive OFFSET greater than the length of SCALAR
+results in the string being padded to the required size with C<"\0">
+bytes before the result of the read is appended.
+
+There is no syseof() function, which is ok, since eof() doesn't work
+well on device files (like ttys) anyway. Use sysread() and check
+for a return value for 0 to decide whether you're done.
+
+Note that if the filehandle has been marked as C<:utf8> Unicode
+characters are read instead of bytes (the LENGTH, OFFSET, and the
+return value of sysread() are in Unicode characters).
+The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
+See L</binmode>, L</open>, and the C<open> pragma, L<open>.
+
+=item sysseek FILEHANDLE,POSITION,WHENCE
+X<sysseek> X<lseek>
+
+Sets FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE may
+be an expression whose value gives the name of the filehandle. The values
+for WHENCE are C<0> to set the new position to POSITION; C<1> to set the it
+to the current position plus POSITION; and C<2> to set it to EOF plus
+POSITION, typically negative.
+
+Note the I<in bytes>: even if the filehandle has been set to operate
+on characters (for example by using the C<:encoding(utf8)> I/O layer),
+tell() will return byte offsets, not character offsets (because
+implementing that would render sysseek() unacceptably slow).
+
+sysseek() bypasses normal buffered IO, so mixing it with reads other
+than C<sysread> (for example C<< <> >> or read()) C<print>, C<write>,
+C<seek>, C<tell>, or C<eof> may cause confusion.
+
+For WHENCE, you may also use the constants C<SEEK_SET>, C<SEEK_CUR>,
+and C<SEEK_END> (start of the file, current position, end of the file)
+from the Fcntl module. Use of the constants is also more portable
+than relying on 0, 1, and 2. For example to define a "systell" function:
+
+ use Fcntl 'SEEK_CUR';
+ sub systell { sysseek($_[0], 0, SEEK_CUR) }
+
+Returns the new position, or the undefined value on failure. A position
+of zero is returned as the string C<"0 but true">; thus C<sysseek> returns
+true on success and false on failure, yet you can still easily determine
+the new position.
+
+=item system LIST
+X<system> X<shell>
+
+=item system PROGRAM LIST
+
+Does exactly the same thing as C<exec LIST>, except that a fork is
+done first and the parent process waits for the child process to
+exit. Note that argument processing varies depending on the
+number of arguments. If there is more than one argument in LIST,
+or if LIST is an array with more than one value, starts the program
+given by the first element of the list with arguments given by the
+rest of the list. If there is only one scalar argument, the argument
+is checked for shell metacharacters, and if there are any, the
+entire argument is passed to the system's command shell for parsing
+(this is C</bin/sh -c> on Unix platforms, but varies on other
+platforms). If there are no shell metacharacters in the argument,
+it is split into words and passed directly to C<execvp>, which is
+more efficient.
+
+Beginning with v5.6.0, Perl will attempt to flush all files opened for
+output before any operation that may do a fork, but this may not be
+supported on some platforms (see L<perlport>). To be safe, you may need
+to set C<$|> ($AUTOFLUSH in English) or call the C<autoflush()> method
+of C<IO::Handle> on any open handles.
+
+The return value is the exit status of the program as returned by the
+C<wait> call. To get the actual exit value, shift right by eight (see
+below). See also L</exec>. This is I<not> what you want to use to capture
+the output from a command; for that you should use merely backticks or
+C<qx//>, as described in L<perlop/"`STRING`">. Return value of -1
+indicates a failure to start the program or an error of the wait(2) system
+call (inspect $! for the reason).
+
+If you'd like to make C<system> (and many other bits of Perl) die on error,
+have a look at the L<autodie> pragma.
+
+Like C<exec>, C<system> allows you to lie to a program about its name if
+you use the C<system PROGRAM LIST> syntax. Again, see L</exec>.
+
+Since C<SIGINT> and C<SIGQUIT> are ignored during the execution of
+C<system>, if you expect your program to terminate on receipt of these
+signals you will need to arrange to do so yourself based on the return
+value.
+
+ @args = ("command", "arg1", "arg2");
+ system(@args) == 0
+ or die "system @args failed: $?"
+
+If you'd like to manually inspect C<system>'s failure, you can check all
+possible failure modes by inspecting C<$?> like this:
+
+ if ($? == -1) {
+ print "failed to execute: $!\n";
+ }
+ elsif ($? & 127) {
+ printf "child died with signal %d, %s coredump\n",
+ ($? & 127), ($? & 128) ? 'with' : 'without';
+ }
+ else {
+ printf "child exited with value %d\n", $? >> 8;
+ }
+
+Alternatively, you may inspect the value of C<${^CHILD_ERROR_NATIVE}>
+with the C<W*()> calls from the POSIX module.
+
+When C<system>'s arguments are executed indirectly by the shell,
+results and return codes are subject to its quirks.
+See L<perlop/"`STRING`"> and L</exec> for details.
+
+Since C<system> does a C<fork> and C<wait> it may affect a C<SIGCHLD>
+handler. See L<perlipc> for details.
+
+Portability issues: L<perlport/system>.
+
+=item syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
+X<syswrite>
+
+=item syswrite FILEHANDLE,SCALAR,LENGTH
+
+=item syswrite FILEHANDLE,SCALAR
+
+Attempts to write LENGTH bytes of data from variable SCALAR to the
+specified FILEHANDLE, using write(2). If LENGTH is
+not specified, writes whole SCALAR. It bypasses buffered IO, so
+mixing this with reads (other than C<sysread())>, C<print>, C<write>,
+C<seek>, C<tell>, or C<eof> may cause confusion because the perlio and
+stdio layers usually buffer data. Returns the number of bytes
+actually written, or C<undef> if there was an error (in this case the
+errno variable C<$!> is also set). If the LENGTH is greater than the
+data available in the SCALAR after the OFFSET, only as much data as is
+available will be written.
+
+An OFFSET may be specified to write the data from some part of the
+string other than the beginning. A negative OFFSET specifies writing
+that many characters counting backwards from the end of the string.
+If SCALAR is of length zero, you can only use an OFFSET of 0.
+
+B<WARNING>: If the filehandle is marked C<:utf8>, Unicode characters
+encoded in UTF-8 are written instead of bytes, and the LENGTH, OFFSET, and
+return value of syswrite() are in (UTF8-encoded Unicode) characters.
+The C<:encoding(...)> layer implicitly introduces the C<:utf8> layer.
+Alternately, if the handle is not marked with an encoding but you
+attempt to write characters with code points over 255, raises an exception.
+See L</binmode>, L</open>, and the C<open> pragma, L<open>.
+
+=item tell FILEHANDLE
+X<tell>
+
+=item tell
+
+Returns the current position I<in bytes> for FILEHANDLE, or -1 on
+error. FILEHANDLE may be an expression whose value gives the name of
+the actual filehandle. If FILEHANDLE is omitted, assumes the file
+last read.
+
+Note the I<in bytes>: even if the filehandle has been set to
+operate on characters (for example by using the C<:encoding(utf8)> open
+layer), tell() will return byte offsets, not character offsets (because
+that would render seek() and tell() rather slow).
+
+The return value of tell() for the standard streams like the STDIN
+depends on the operating system: it may return -1 or something else.
+tell() on pipes, fifos, and sockets usually returns -1.
+
+There is no C<systell> function. Use C<sysseek(FH, 0, 1)> for that.
+
+Do not use tell() (or other buffered I/O operations) on a filehandle
+that has been manipulated by sysread(), syswrite(), or sysseek().
+Those functions ignore the buffering, while tell() does not.
+
+=item telldir DIRHANDLE
+X<telldir>
+
+Returns the current position of the C<readdir> routines on DIRHANDLE.
+Value may be given to C<seekdir> to access a particular location in a
+directory. C<telldir> has the same caveats about possible directory
+compaction as the corresponding system library routine.
+
+=item tie VARIABLE,CLASSNAME,LIST
+X<tie>
+
+This function binds a variable to a package class that will provide the
+implementation for the variable. VARIABLE is the name of the variable
+to be enchanted. CLASSNAME is the name of a class implementing objects
+of correct type. Any additional arguments are passed to the C<new>
+method of the class (meaning C<TIESCALAR>, C<TIEHANDLE>, C<TIEARRAY>,
+or C<TIEHASH>). Typically these are arguments such as might be passed
+to the C<dbm_open()> function of C. The object returned by the C<new>
+method is also returned by the C<tie> function, which would be useful
+if you want to access other methods in CLASSNAME.
+
+Note that functions such as C<keys> and C<values> may return huge lists
+when used on large objects, like DBM files. You may prefer to use the
+C<each> function to iterate over such. Example:
+
+ # print out history file offsets
+ use NDBM_File;
+ tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
+ while (($key,$val) = each %HIST) {
+ print $key, ' = ', unpack('L',$val), "\n";
+ }
+ untie(%HIST);
+
+A class implementing a hash should have the following methods:
+
+ TIEHASH classname, LIST
+ FETCH this, key
+ STORE this, key, value
+ DELETE this, key
+ CLEAR this
+ EXISTS this, key
+ FIRSTKEY this
+ NEXTKEY this, lastkey
+ SCALAR this
+ DESTROY this
+ UNTIE this
+
+A class implementing an ordinary array should have the following methods:
+
+ TIEARRAY classname, LIST
+ FETCH this, key
+ STORE this, key, value
+ FETCHSIZE this
+ STORESIZE this, count
+ CLEAR this
+ PUSH this, LIST
+ POP this
+ SHIFT this
+ UNSHIFT this, LIST
+ SPLICE this, offset, length, LIST
+ EXTEND this, count
+ DESTROY this
+ UNTIE this
+
+A class implementing a filehandle should have the following methods:
+
+ TIEHANDLE classname, LIST
+ READ this, scalar, length, offset
+ READLINE this
+ GETC this
+ WRITE this, scalar, length, offset
+ PRINT this, LIST
+ PRINTF this, format, LIST
+ BINMODE this
+ EOF this
+ FILENO this
+ SEEK this, position, whence
+ TELL this
+ OPEN this, mode, LIST
+ CLOSE this
+ DESTROY this
+ UNTIE this
+
+A class implementing a scalar should have the following methods:
+
+ TIESCALAR classname, LIST
+ FETCH this,
+ STORE this, value
+ DESTROY this
+ UNTIE this
+
+Not all methods indicated above need be implemented. See L<perltie>,
+L<Tie::Hash>, L<Tie::Array>, L<Tie::Scalar>, and L<Tie::Handle>.
+
+Unlike C<dbmopen>, the C<tie> function will not C<use> or C<require> a module
+for you; you need to do that explicitly yourself. See L<DB_File>
+or the F<Config> module for interesting C<tie> implementations.
+
+For further details see L<perltie>, L<"tied VARIABLE">.
+
+=item tied VARIABLE
+X<tied>
+
+Returns a reference to the object underlying VARIABLE (the same value
+that was originally returned by the C<tie> call that bound the variable
+to a package.) Returns the undefined value if VARIABLE isn't tied to a
+package.
+
+=item time
+X<time> X<epoch>
+
+Returns the number of non-leap seconds since whatever time the system
+considers to be the epoch, suitable for feeding to C<gmtime> and
+C<localtime>. On most systems the epoch is 00:00:00 UTC, January 1, 1970;
+a prominent exception being Mac OS Classic which uses 00:00:00, January 1,
+1904 in the current local time zone for its epoch.
+
+For measuring time in better granularity than one second, use the
+L<Time::HiRes> module from Perl 5.8 onwards (or from CPAN before then), or,
+if you have gettimeofday(2), you may be able to use the C<syscall>
+interface of Perl. See L<perlfaq8> for details.
+
+For date and time processing look at the many related modules on CPAN.
+For a comprehensive date and time representation look at the
+L<DateTime> module.
+
+=item times
+X<times>
+
+Returns a four-element list giving the user and system times in
+seconds for this process and any exited children of this process.
+
+ ($user,$system,$cuser,$csystem) = times;
+
+In scalar context, C<times> returns C<$user>.
+
+Children's times are only included for terminated children.
+
+Portability issues: L<perlport/times>.
+
+=item tr///
+
+The transliteration operator. Same as C<y///>. See
+L<perlop/"Quote and Quote-like Operators">.
+
+=item truncate FILEHANDLE,LENGTH
+X<truncate>
+
+=item truncate EXPR,LENGTH
+
+Truncates the file opened on FILEHANDLE, or named by EXPR, to the
+specified length. Raises an exception if truncate isn't implemented
+on your system. Returns true if successful, C<undef> on error.
+
+The behavior is undefined if LENGTH is greater than the length of the
+file.
+
+The position in the file of FILEHANDLE is left unchanged. You may want to
+call L<seek|/"seek FILEHANDLE,POSITION,WHENCE"> before writing to the file.
+
+Portability issues: L<perlport/truncate>.
+
+=item uc EXPR
+X<uc> X<uppercase> X<toupper>
+
+=item uc
+
+Returns an uppercased version of EXPR. This is the internal function
+implementing the C<\U> escape in double-quoted strings.
+It does not attempt to do titlecase mapping on initial letters. See
+L</ucfirst> for that.
+
+If EXPR is omitted, uses C<$_>.
+
+This function behaves the same way under various pragma, such as in a locale,
+as L</lc> does.
+
+=item ucfirst EXPR
+X<ucfirst> X<uppercase>
+
+=item ucfirst
+
+Returns the value of EXPR with the first character in uppercase
+(titlecase in Unicode). This is the internal function implementing
+the C<\u> escape in double-quoted strings.
+
+If EXPR is omitted, uses C<$_>.
+
+This function behaves the same way under various pragma, such as in a locale,
+as L</lc> does.
+
+=item umask EXPR
+X<umask>
+
+=item umask
+
+Sets the umask for the process to EXPR and returns the previous value.
+If EXPR is omitted, merely returns the current umask.
+
+The Unix permission C<rwxr-x---> is represented as three sets of three
+bits, or three octal digits: C<0750> (the leading 0 indicates octal
+and isn't one of the digits). The C<umask> value is such a number
+representing disabled permissions bits. The permission (or "mode")
+values you pass C<mkdir> or C<sysopen> are modified by your umask, so
+even if you tell C<sysopen> to create a file with permissions C<0777>,
+if your umask is C<0022>, then the file will actually be created with
+permissions C<0755>. If your C<umask> were C<0027> (group can't
+write; others can't read, write, or execute), then passing
+C<sysopen> C<0666> would create a file with mode C<0640> (because
+C<0666 &~ 027> is C<0640>).
+
+Here's some advice: supply a creation mode of C<0666> for regular
+files (in C<sysopen>) and one of C<0777> for directories (in
+C<mkdir>) and executable files. This gives users the freedom of
+choice: if they want protected files, they might choose process umasks
+of C<022>, C<027>, or even the particularly antisocial mask of C<077>.
+Programs should rarely if ever make policy decisions better left to
+the user. The exception to this is when writing files that should be
+kept private: mail files, web browser cookies, I<.rhosts> files, and
+so on.
+
+If umask(2) is not implemented on your system and you are trying to
+restrict access for I<yourself> (i.e., C<< (EXPR & 0700) > 0 >>),
+raises an exception. If umask(2) is not implemented and you are
+not trying to restrict access for yourself, returns C<undef>.
+
+Remember that a umask is a number, usually given in octal; it is I<not> a
+string of octal digits. See also L</oct>, if all you have is a string.
+
+Portability issues: L<perlport/umask>.
+
+=item undef EXPR
+X<undef> X<undefine>
+
+=item undef
+
+Undefines the value of EXPR, which must be an lvalue. Use only on a
+scalar value, an array (using C<@>), a hash (using C<%>), a subroutine
+(using C<&>), or a typeglob (using C<*>). Saying C<undef $hash{$key}>
+will probably not do what you expect on most predefined variables or
+DBM list values, so don't do that; see L</delete>. Always returns the
+undefined value. You can omit the EXPR, in which case nothing is
+undefined, but you still get an undefined value that you could, for
+instance, return from a subroutine, assign to a variable, or pass as a
+parameter. Examples:
+
+ undef $foo;
+ undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
+ undef @ary;
+ undef %hash;
+ undef &mysub;
+ undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
+ return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
+ select undef, undef, undef, 0.25;
+ ($a, $b, undef, $c) = &foo; # Ignore third value returned
+
+Note that this is a unary operator, not a list operator.
+
+=item unlink LIST
+X<unlink> X<delete> X<remove> X<rm> X<del>
+
+=item unlink
+
+Deletes a list of files. On success, it returns the number of files
+it successfully deleted. On failure, it returns false and sets C<$!>
+(errno):
+
+ my $unlinked = unlink 'a', 'b', 'c';
+ unlink @goners;
+ unlink glob "*.bak";
+
+On error, C<unlink> will not tell you which files it could not remove.
+If you want to know which files you could not remove, try them one
+at a time:
+
+ foreach my $file ( @goners ) {
+ unlink $file or warn "Could not unlink $file: $!";
+ }
+
+Note: C<unlink> will not attempt to delete directories unless you are
+superuser and the B<-U> flag is supplied to Perl. Even if these
+conditions are met, be warned that unlinking a directory can inflict
+damage on your filesystem. Finally, using C<unlink> on directories is
+not supported on many operating systems. Use C<rmdir> instead.
+
+If LIST is omitted, C<unlink> uses C<$_>.
+
+=item unpack TEMPLATE,EXPR
+X<unpack>
+
+=item unpack TEMPLATE
+
+C<unpack> does the reverse of C<pack>: it takes a string
+and expands it out into a list of values.
+(In scalar context, it returns merely the first value produced.)
+
+If EXPR is omitted, unpacks the C<$_> string.
+See L<perlpacktut> for an introduction to this function.
+
+The string is broken into chunks described by the TEMPLATE. Each chunk
+is converted separately to a value. Typically, either the string is a result
+of C<pack>, or the characters of the string represent a C structure of some
+kind.
+
+The TEMPLATE has the same format as in the C<pack> function.
+Here's a subroutine that does substring:
+
+ sub substr {
+ my($what,$where,$howmuch) = @_;
+ unpack("x$where a$howmuch", $what);
+ }
+
+and then there's
+
+ sub ordinal { unpack("W",$_[0]); } # same as ord()
+
+In addition to fields allowed in pack(), you may prefix a field with
+a %<number> to indicate that
+you want a <number>-bit checksum of the items instead of the items
+themselves. Default is a 16-bit checksum. Checksum is calculated by
+summing numeric values of expanded values (for string fields the sum of
+C<ord($char)> is taken; for bit fields the sum of zeroes and ones).
+
+For example, the following
+computes the same number as the System V sum program:
+
+ $checksum = do {
+ local $/; # slurp!
+ unpack("%32W*",<>) % 65535;
+ };
+
+The following efficiently counts the number of set bits in a bit vector:
+
+ $setbits = unpack("%32b*", $selectmask);
+
+The C<p> and C<P> formats should be used with care. Since Perl
+has no way of checking whether the value passed to C<unpack()>
+corresponds to a valid memory location, passing a pointer value that's
+not known to be valid is likely to have disastrous consequences.
+
+If there are more pack codes or if the repeat count of a field or a group
+is larger than what the remainder of the input string allows, the result
+is not well defined: the repeat count may be decreased, or
+C<unpack()> may produce empty strings or zeros, or it may raise an exception.
+If the input string is longer than one described by the TEMPLATE,
+the remainder of that input string is ignored.
+
+See L</pack> for more examples and notes.
+
+=item untie VARIABLE
+X<untie>
+
+Breaks the binding between a variable and a package.
+(See L<tie|/tie VARIABLE,CLASSNAME,LIST>.)
+Has no effect if the variable is not tied.
+
+=item unshift ARRAY,LIST
+X<unshift>
+
+=item unshift EXPR,LIST
+
+Does the opposite of a C<shift>. Or the opposite of a C<push>,
+depending on how you look at it. Prepends list to the front of the
+array and returns the new number of elements in the array.
+
+ unshift(@ARGV, '-e') unless $ARGV[0] =~ /^-/;
+
+Note the LIST is prepended whole, not one element at a time, so the
+prepended elements stay in the same order. Use C<reverse> to do the
+reverse.
+
+Starting with Perl 5.14, C<unshift> can take a scalar EXPR, which must hold
+a reference to an unblessed array. The argument will be dereferenced
+automatically. This aspect of C<unshift> is considered highly
+experimental. The exact behaviour may change in a future version of Perl.
+
+=item use Module VERSION LIST
+X<use> X<module> X<import>
+
+=item use Module VERSION
+
+=item use Module LIST
+
+=item use Module
+
+=item use VERSION
+
+Imports some semantics into the current package from the named module,
+generally by aliasing certain subroutine or variable names into your
+package. It is exactly equivalent to
+
+ BEGIN { require Module; Module->import( LIST ); }
+
+except that Module I<must> be a bareword.
+The importation can be made conditional; see L<if>.
+
+In the peculiar C<use VERSION> form, VERSION may be either a positive
+decimal fraction such as 5.006, which will be compared to C<$]>, or a v-string
+of the form v5.6.1, which will be compared to C<$^V> (aka $PERL_VERSION). An
+exception is raised if VERSION is greater than the version of the
+current Perl interpreter; Perl will not attempt to parse the rest of the
+file. Compare with L</require>, which can do a similar check at run time.
+Symmetrically, C<no VERSION> allows you to specify that you want a version
+of Perl older than the specified one.
+
+Specifying VERSION as a literal of the form v5.6.1 should generally be
+avoided, because it leads to misleading error messages under earlier
+versions of Perl (that is, prior to 5.6.0) that do not support this
+syntax. The equivalent numeric version should be used instead.
+
+ use v5.6.1; # compile time version check
+ use 5.6.1; # ditto
+ use 5.006_001; # ditto; preferred for backwards compatibility
+
+This is often useful if you need to check the current Perl version before
+C<use>ing library modules that won't work with older versions of Perl.
+(We try not to do this more than we have to.)
+
+Also, if the specified Perl version is greater than or equal to 5.9.5,
+C<use VERSION> will also load the C<feature> pragma and enable all
+features available in the requested version. See L<feature>.
+Similarly, if the specified Perl version is greater than or equal to
+5.11.0, strictures are enabled lexically as with C<use strict> (except
+that the F<strict.pm> file is not actually loaded).
+
+The C<BEGIN> forces the C<require> and C<import> to happen at compile time. The
+C<require> makes sure the module is loaded into memory if it hasn't been
+yet. The C<import> is not a builtin; it's just an ordinary static method
+call into the C<Module> package to tell the module to import the list of
+features back into the current package. The module can implement its
+C<import> method any way it likes, though most modules just choose to
+derive their C<import> method via inheritance from the C<Exporter> class that
+is defined in the C<Exporter> module. See L<Exporter>. If no C<import>
+method can be found then the call is skipped, even if there is an AUTOLOAD
+method.
+
+If you do not want to call the package's C<import> method (for instance,
+to stop your namespace from being altered), explicitly supply the empty list:
+
+ use Module ();
+
+That is exactly equivalent to
+
+ BEGIN { require Module }
+
+If the VERSION argument is present between Module and LIST, then the
+C<use> will call the VERSION method in class Module with the given
+version as an argument. The default VERSION method, inherited from
+the UNIVERSAL class, croaks if the given version is larger than the
+value of the variable C<$Module::VERSION>.
+
+Again, there is a distinction between omitting LIST (C<import> called
+with no arguments) and an explicit empty LIST C<()> (C<import> not
+called). Note that there is no comma after VERSION!
+
+Because this is a wide-open interface, pragmas (compiler directives)
+are also implemented this way. Currently implemented pragmas are:
+
+ use constant;
+ use diagnostics;
+ use integer;
+ use sigtrap qw(SEGV BUS);
+ use strict qw(subs vars refs);
+ use subs qw(afunc blurfl);
+ use warnings qw(all);
+ use sort qw(stable _quicksort _mergesort);
+
+Some of these pseudo-modules import semantics into the current
+block scope (like C<strict> or C<integer>, unlike ordinary modules,
+which import symbols into the current package (which are effective
+through the end of the file).
+
+Because C<use> takes effect at compile time, it doesn't respect the
+ordinary flow control of the code being compiled. In particular, putting
+a C<use> inside the false branch of a conditional doesn't prevent it
+from being processed. If a module or pragma only needs to be loaded
+conditionally, this can be done using the L<if> pragma:
+
+ use if $] < 5.008, "utf8";
+ use if WANT_WARNINGS, warnings => qw(all);
+
+There's a corresponding C<no> declaration that unimports meanings imported
+by C<use>, i.e., it calls C<unimport Module LIST> instead of C<import>.
+It behaves just as C<import> does with VERSION, an omitted or empty LIST,
+or no unimport method being found.
+
+ no integer;
+ no strict 'refs';
+ no warnings;
+
+Care should be taken when using the C<no VERSION> form of C<no>. It is
+I<only> meant to be used to assert that the running Perl is of a earlier
+version than its argument and I<not> to undo the feature-enabling side effects
+of C<use VERSION>.
+
+See L<perlmodlib> for a list of standard modules and pragmas. See L<perlrun>
+for the C<-M> and C<-m> command-line options to Perl that give C<use>
+functionality from the command-line.
+
+=item utime LIST
+X<utime>
+
+Changes the access and modification times on each file of a list of
+files. The first two elements of the list must be the NUMERIC access
+and modification times, in that order. Returns the number of files
+successfully changed. The inode change time of each file is set
+to the current time. For example, this code has the same effect as the
+Unix touch(1) command when the files I<already exist> and belong to
+the user running the program:
+
+ #!/usr/bin/perl
+ $atime = $mtime = time;
+ utime $atime, $mtime, @ARGV;
+
+Since Perl 5.7.2, if the first two elements of the list are C<undef>,
+the utime(2) syscall from your C library is called with a null second
+argument. On most systems, this will set the file's access and
+modification times to the current time (i.e., equivalent to the example
+above) and will work even on files you don't own provided you have write
+permission:
+
+ for $file (@ARGV) {
+ utime(undef, undef, $file)
+ || warn "couldn't touch $file: $!";
+ }
+
+Under NFS this will use the time of the NFS server, not the time of
+the local machine. If there is a time synchronization problem, the
+NFS server and local machine will have different times. The Unix
+touch(1) command will in fact normally use this form instead of the
+one shown in the first example.
+
+Passing only one of the first two elements as C<undef> is
+equivalent to passing a 0 and will not have the effect
+described when both are C<undef>. This also triggers an
+uninitialized warning.
+
+On systems that support futimes(2), you may pass filehandles among the
+files. On systems that don't support futimes(2), passing filehandles raises
+an exception. Filehandles must be passed as globs or glob references to be
+recognized; barewords are considered filenames.
+
+Portability issues: L<perlport/utime>.
+
+=item values HASH
+X<values>
+
+=item values ARRAY
+
+=item values EXPR
+
+Returns a list consisting of all the values of the named hash, or the values
+of an array. (In scalar context, returns the number of values.)
+
+The values are returned in an apparently random order. The actual
+random order is subject to change in future versions of Perl, but it
+is guaranteed to be the same order as either the C<keys> or C<each>
+function would produce on the same (unmodified) hash. Since Perl
+5.8.1 the ordering is different even between different runs of Perl
+for security reasons (see L<perlsec/"Algorithmic Complexity Attacks">).
+
+As a side effect, calling values() resets the HASH or ARRAY's internal
+iterator;
+see L</each>. (In particular, calling values() in void context resets
+the iterator with no other overhead. Apart from resetting the iterator,
+C<values @array> in list context is the same as plain C<@array>.
+We recommend that you use void context C<keys @array> for this, but reasoned
+that it taking C<values @array> out would require more documentation than
+leaving it in.)
+
+Note that the values are not copied, which means modifying them will
+modify the contents of the hash:
+
+ for (values %hash) { s/foo/bar/g } # modifies %hash values
+ for (@hash{keys %hash}) { s/foo/bar/g } # same
+
+Starting with Perl 5.14, C<values> can take a scalar EXPR, which must hold
+a reference to an unblessed hash or array. The argument will be
+dereferenced automatically. This aspect of C<values> is considered highly
+experimental. The exact behaviour may change in a future version of Perl.
+
+ for (values $hashref) { ... }
+ for (values $obj->get_arrayref) { ... }
+
+See also C<keys>, C<each>, and C<sort>.
+
+=item vec EXPR,OFFSET,BITS
+X<vec> X<bit> X<bit vector>
+
+Treats the string in EXPR as a bit vector made up of elements of
+width BITS and returns the value of the element specified by OFFSET
+as an unsigned integer. BITS therefore specifies the number of bits
+that are reserved for each element in the bit vector. This must
+be a power of two from 1 to 32 (or 64, if your platform supports
+that).
+
+If BITS is 8, "elements" coincide with bytes of the input string.
+
+If BITS is 16 or more, bytes of the input string are grouped into chunks
+of size BITS/8, and each group is converted to a number as with
+pack()/unpack() with big-endian formats C<n>/C<N> (and analogously
+for BITS==64). See L<"pack"> for details.
+
+If bits is 4 or less, the string is broken into bytes, then the bits
+of each byte are broken into 8/BITS groups. Bits of a byte are
+numbered in a little-endian-ish way, as in C<0x01>, C<0x02>,
+C<0x04>, C<0x08>, C<0x10>, C<0x20>, C<0x40>, C<0x80>. For example,
+breaking the single input byte C<chr(0x36)> into two groups gives a list
+C<(0x6, 0x3)>; breaking it into 4 groups gives C<(0x2, 0x1, 0x3, 0x0)>.
+
+C<vec> may also be assigned to, in which case parentheses are needed
+to give the expression the correct precedence as in
+
+ vec($image, $max_x * $x + $y, 8) = 3;
+
+If the selected element is outside the string, the value 0 is returned.
+If an element off the end of the string is written to, Perl will first
+extend the string with sufficiently many zero bytes. It is an error
+to try to write off the beginning of the string (i.e., negative OFFSET).
+
+If the string happens to be encoded as UTF-8 internally (and thus has
+the UTF8 flag set), this is ignored by C<vec>, and it operates on the
+internal byte string, not the conceptual character string, even if you
+only have characters with values less than 256.
+
+Strings created with C<vec> can also be manipulated with the logical
+operators C<|>, C<&>, C<^>, and C<~>. These operators will assume a bit
+vector operation is desired when both operands are strings.
+See L<perlop/"Bitwise String Operators">.
+
+The following code will build up an ASCII string saying C<'PerlPerlPerl'>.
+The comments show the string after each step. Note that this code works
+in the same way on big-endian or little-endian machines.
+
+ my $foo = '';
+ vec($foo, 0, 32) = 0x5065726C; # 'Perl'
+
+ # $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
+ print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
+
+ vec($foo, 2, 16) = 0x5065; # 'PerlPe'
+ vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
+ vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
+ vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
+ vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
+ vec($foo, 21, 4) = 7; # 'PerlPerlPer'
+ # 'r' is "\x72"
+ vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
+ vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
+ vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
+ # 'l' is "\x6c"
+
+To transform a bit vector into a string or list of 0's and 1's, use these:
+
+ $bits = unpack("b*", $vector);
+ @bits = split(//, unpack("b*", $vector));
+
+If you know the exact length in bits, it can be used in place of the C<*>.
+
+Here is an example to illustrate how the bits actually fall in place:
+
+ #!/usr/bin/perl -wl
+
+ print <<'EOT';
+ 0 1 2 3
+ unpack("V",$_) 01234567890123456789012345678901
+ ------------------------------------------------------------------
+ EOT
+
+ for $w (0..3) {
+ $width = 2**$w;
+ for ($shift=0; $shift < $width; ++$shift) {
+ for ($off=0; $off < 32/$width; ++$off) {
+ $str = pack("B*", "0"x32);
+ $bits = (1<<$shift);
+ vec($str, $off, $width) = $bits;
+ $res = unpack("b*",$str);
+ $val = unpack("V", $str);
+ write;
+ }
+ }
+ }
+
+ format STDOUT =
+ vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
+ $off, $width, $bits, $val, $res
+ .
+ __END__
+
+Regardless of the machine architecture on which it runs, the
+example above should print the following table:
+
+ 0 1 2 3
+ unpack("V",$_) 01234567890123456789012345678901
+ ------------------------------------------------------------------
+ vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
+ vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
+ vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
+ vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
+ vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
+ vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
+ vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
+ vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
+ vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
+ vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
+ vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
+ vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
+ vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
+ vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
+ vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
+ vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
+ vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
+ vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
+ vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
+ vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
+ vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
+ vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
+ vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
+ vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
+ vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
+ vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
+ vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
+ vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
+ vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
+ vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
+ vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
+ vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
+ vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
+ vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
+ vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
+ vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
+ vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
+ vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
+ vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
+ vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
+ vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
+ vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
+ vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
+ vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
+ vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
+ vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
+ vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
+ vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
+ vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
+ vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
+ vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
+ vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
+ vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
+ vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
+ vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
+ vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
+ vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
+ vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
+ vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
+ vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
+ vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
+ vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
+ vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
+ vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
+ vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
+ vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
+ vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
+ vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
+ vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
+ vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
+ vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
+ vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
+ vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
+ vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
+ vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
+ vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
+ vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
+ vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
+ vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
+ vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
+ vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
+ vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
+ vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
+ vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
+ vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
+ vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
+ vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
+ vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
+ vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
+ vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
+ vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
+ vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
+ vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
+ vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
+ vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
+ vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
+ vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
+ vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
+ vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
+ vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
+ vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
+ vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
+ vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
+ vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
+ vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
+ vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
+ vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
+ vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
+ vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
+ vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
+ vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
+ vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
+ vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
+ vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
+ vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
+ vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
+ vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
+ vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
+ vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
+ vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
+ vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
+ vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
+ vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
+ vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
+ vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
+ vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
+ vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
+ vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001
+
+=item wait
+X<wait>
+
+Behaves like wait(2) on your system: it waits for a child
+process to terminate and returns the pid of the deceased process, or
+C<-1> if there are no child processes. The status is returned in C<$?>
+and C<${^CHILD_ERROR_NATIVE}>.
+Note that a return value of C<-1> could mean that child processes are
+being automatically reaped, as described in L<perlipc>.
+
+If you use wait in your handler for $SIG{CHLD} it may accidentally for the
+child created by qx() or system(). See L<perlipc> for details.
+
+Portability issues: L<perlport/wait>.
+
+=item waitpid PID,FLAGS
+X<waitpid>
+
+Waits for a particular child process to terminate and returns the pid of
+the deceased process, or C<-1> if there is no such child process. On some
+systems, a value of 0 indicates that there are processes still running.
+The status is returned in C<$?> and C<${^CHILD_ERROR_NATIVE}>. If you say
+
+ use POSIX ":sys_wait_h";
+ #...
+ do {
+ $kid = waitpid(-1, WNOHANG);
+ } while $kid > 0;
+
+then you can do a non-blocking wait for all pending zombie processes.
+Non-blocking wait is available on machines supporting either the
+waitpid(2) or wait4(2) syscalls. However, waiting for a particular
+pid with FLAGS of C<0> is implemented everywhere. (Perl emulates the
+system call by remembering the status values of processes that have
+exited but have not been harvested by the Perl script yet.)
+
+Note that on some systems, a return value of C<-1> could mean that child
+processes are being automatically reaped. See L<perlipc> for details,
+and for other examples.
+
+Portability issues: L<perlport/waitpid>.
+
+=item wantarray
+X<wantarray> X<context>
+
+Returns true if the context of the currently executing subroutine or
+C<eval> is looking for a list value. Returns false if the context is
+looking for a scalar. Returns the undefined value if the context is
+looking for no value (void context).
+
+ return unless defined wantarray; # don't bother doing more
+ my @a = complex_calculation();
+ return wantarray ? @a : "@a";
+
+C<wantarray()>'s result is unspecified in the top level of a file,
+in a C<BEGIN>, C<UNITCHECK>, C<CHECK>, C<INIT> or C<END> block, or
+in a C<DESTROY> method.
+
+This function should have been named wantlist() instead.
+
+=item warn LIST
+X<warn> X<warning> X<STDERR>
+
+Prints the value of LIST to STDERR. If the last element of LIST does
+not end in a newline, it appends the same file/line number text as C<die>
+does.
+
+If the output is empty and C<$@> already contains a value (typically from a
+previous eval) that value is used after appending C<"\t...caught">
+to C<$@>. This is useful for staying almost, but not entirely similar to
+C<die>.
+
+If C<$@> is empty then the string C<"Warning: Something's wrong"> is used.
+
+No message is printed if there is a C<$SIG{__WARN__}> handler
+installed. It is the handler's responsibility to deal with the message
+as it sees fit (like, for instance, converting it into a C<die>). Most
+handlers must therefore arrange to actually display the
+warnings that they are not prepared to deal with, by calling C<warn>
+again in the handler. Note that this is quite safe and will not
+produce an endless loop, since C<__WARN__> hooks are not called from
+inside one.
+
+You will find this behavior is slightly different from that of
+C<$SIG{__DIE__}> handlers (which don't suppress the error text, but can
+instead call C<die> again to change it).
+
+Using a C<__WARN__> handler provides a powerful way to silence all
+warnings (even the so-called mandatory ones). An example:
+
+ # wipe out *all* compile-time warnings
+ BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
+ my $foo = 10;
+ my $foo = 20; # no warning about duplicate my $foo,
+ # but hey, you asked for it!
+ # no compile-time or run-time warnings before here
+ $DOWARN = 1;
+
+ # run-time warnings enabled after here
+ warn "\$foo is alive and $foo!"; # does show up
+
+See L<perlvar> for details on setting C<%SIG> entries and for more
+examples. See the Carp module for other kinds of warnings using its
+carp() and cluck() functions.
+
+=item when EXPR BLOCK
+X<when>
+
+=item when BLOCK
+
+C<when> is analogous to the C<case> keyword in other languages. Used with a
+C<foreach> loop or the experimental C<given> block, C<when> can be used in
+Perl to implement C<switch>/C<case> like statements. Available as a
+statement after Perl 5.10 and as a statement modifier after 5.14.
+Here are three examples:
+
+ use v5.10;
+ foreach (@fruits) {
+ when (/apples?/) {
+ say "I like apples."
+ }
+ when (/oranges?/) {
+ say "I don't like oranges."
+ }
+ default {
+ say "I don't like anything"
+ }
+ }
+
+ # require 5.14 for when as statement modifier
+ use v5.14;
+ foreach (@fruits) {
+ say "I like apples." when /apples?/;
+ say "I don't like oranges." when /oranges?;
+ default { say "I don't like anything" }
+ }
+
+ use v5.10;
+ given ($fruit) {
+ when (/apples?/) {
+ say "I like apples."
+ }
+ when (/oranges?/) {
+ say "I don't like oranges."
+ }
+ default {
+ say "I don't like anything"
+ }
+ }
+
+See L<perlsyn/"Switch statements"> for detailed information.
+
+=item write FILEHANDLE
+X<write>
+
+=item write EXPR
+
+=item write
+
+Writes a formatted record (possibly multi-line) to the specified FILEHANDLE,
+using the format associated with that file. By default the format for
+a file is the one having the same name as the filehandle, but the
+format for the current output channel (see the C<select> function) may be set
+explicitly by assigning the name of the format to the C<$~> variable.
+
+Top of form processing is handled automatically: if there is insufficient
+room on the current page for the formatted record, the page is advanced by
+writing a form feed, a special top-of-page format is used to format the new
+page header before the record is written. By default, the top-of-page
+format is the name of the filehandle with "_TOP" appended. This would be a
+problem with autovivified filehandles, but it may be dynamically set to the
+format of your choice by assigning the name to the C<$^> variable while
+that filehandle is selected. The number of lines remaining on the current
+page is in variable C<$->, which can be set to C<0> to force a new page.
+
+If FILEHANDLE is unspecified, output goes to the current default output
+channel, which starts out as STDOUT but may be changed by the
+C<select> operator. If the FILEHANDLE is an EXPR, then the expression
+is evaluated and the resulting string is used to look up the name of
+the FILEHANDLE at run time. For more on formats, see L<perlform>.
+
+Note that write is I<not> the opposite of C<read>. Unfortunately.
+
+=item y///
+
+The transliteration operator. Same as C<tr///>. See
+L<perlop/"Quote and Quote-like Operators">.
+
+=back
+
+=cut
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