more perldiag grammar/punctuation tweaks

[perl5.git] / pod / perlfunc.pod

=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 a 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 a scalar context by
returning the undefined value, and in a 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.) all 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 are 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<exit>,
C<goto>, C<last>, C<next>, C<redo>, C<return>, C<sub>, C<wantarray>

=item Keywords related to switch

C<break>, C<continue>, C<given>, C<when>, C<default>

(These are available only if you enable the C<"switch"> feature.
See L<feature> and L<perlsyn/"Switch statements">.)

=item Keywords related to scoping

C<caller>, C<import>, C<local>, C<my>, C<our>, C<state>, C<package>,
C<use>

(C<state> is available only if the C<"state"> feature is enabled. See
L<feature>.)

=item Miscellaneous functions

C<defined>, C<dump>, C<eval>, 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<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
        #...
    }

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 the 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.

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.

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> operators) are 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.)

=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>.

=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.

=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 to always use it when appropriate,
and to never use it when it isn't appropriate.  Also, people can
set their I/O to be by default UTF-8 encoded Unicode, not bytes.

In other words: regardless of platform, use binmode() on binary data,
like for example images.

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(utf8)>.
C<:utf8> just marks the data as UTF-8 without further checking,
while C<:encoding(utf8)> 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 work together to let the programmer treat a single
character (C<\n>) as the line terminator, irrespective of the 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.

Mac OS, all variants of Unix, 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 Mac OS and 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 contains 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.

=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.
See L<perltoot> and L<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.

=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.

Also 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 means that the effects of C<shift @_> can
I<normally> be undone (but not C<pop @_> or other splicing, and not if a
reference to C<@_> has been taken, 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 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 numerical 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.

=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);

=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<$_>.

=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 have succeeded 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
C<open>.)  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}>.

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.

=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

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.

If the C<"switch"> feature is enabled, C<continue> is also a
function that exits the current C<when> (or C<default>) block and
falls through to the next one.  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 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 is 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, do not assume
anything about the returned string itself, or how many bytes in the
digest 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)
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>.

=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.

=item dbmopen HASH,DBNAME,MASK
X<dbmopen> X<dbm> X<ndbm> X<sdbm> X<gdbm>

[This function has been largely superseded by the C<tie> 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: $!";

=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 has 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 then are 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>.

It returns the value or values deleted in list context, 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<Be aware> that 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{expr}> 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.  See L<perlsub>.

=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 @INC directories, and updates
C<%INC> if the file is found.  See L<perlvar/Predefined Names> 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> cannot read the file, it returns undef and sets C<$!> to the
error.  If C<do> can read the file but cannot compile it, it
returns undef and sets an error message in C<$@>.   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.

=item each HASH (or HASHREF)
X<each> X<hash, iterator>

=item each ARRAY (or ARRAYREF)
X<array, iterator>

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";
    }

When given a reference to a hash or array, the argument will be
dereferenced automatically.

    while (($key,$value) = each $hashref) { ... }

If the reference is a blessed object that overrides either C<%{}> or
C<@{}>, the override will be used instead of dereferencing the underlying
variable type.  If both overrides are provided, C<%{}> will be the default.
If this is not desired, you must dereference the argument yourself.

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, 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, C<eof()> will detect the end of only the
last file.  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 if
there was 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 weren't any
errors, executed in the lexical context of the current Perl program, so
that any 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.

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 an undefined value in scalar context
or an empty list in list context, and C<$@> is set to the
error message.  If there was no error, C<$@> is guaranteed to be the empty
string.  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<$@> occured before restoration of localised variables, which means that, if your code is 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 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.

=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 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 &sub;    # 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.

=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";

=item fileno FILEHANDLE
X<fileno>

Returns the file descriptor for a filehandle, or undefined if the
filehandle is not open.  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";
    }

(Filehandles connected to memory objects via new features of C<open> may
return undefined even though they are open.)


=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
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, or 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 Fcntl module,
either individually, or as a group using the ':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
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.

=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.

=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.

=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 on
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>.

=item getpeername SOCKET
X<getpeername> X<peer>

Returns the packed sockaddr address of 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 process
group of current process.  Note that the POSIX version of C<getpgrp>
does not accept a PID argument, so only C<PID==0> is truly portable.

=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>.

=item getpriority WHICH,WHO
X<getpriority> X<priority> X<nice>

Returns the current priority for a process, a process group, or a user.
(See C<getpriority(2)>.)  Will raise a fatal exception if used on a
machine that doesn't implement getpriority(2).

=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, because of 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 your getpwnam(3) documentation and your
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 <gethostbyname()> is called in SCALAR context and that
its return value is checked for definedness.

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 like they're the same method calls (uid),
they aren't, because a C<File::stat> object is different from
a C<User::pwent> object.

=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.

An example to test whether Nagle's algorithm is turned on 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";


=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 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.

=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.

See L<perlport/gmtime> for portability concerns.

=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];

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>, or 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 C<0> (or whatever
you've set the C<$[> variable to--but don't do that).  If the substring
is not found, C<index> returns one less than the base, ordinarily C<-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.

=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 (or HASHREF)
X<keys> X<key>

=item keys ARRAY (or ARRAYREF)

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 HASH or ARRAY's internal iterator
(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.

When given a reference to a hash or array, the argument will be
dereferenced automatically.

    for (keys $hashref) { ... }
    for (keys $obj->get_arrayref) { ... }

If the reference is a blessed object that overrides either C<%{}> or
C<@{}>, the override will be used instead of dereferencing the underlying
variable type.  If both overrides are provided, C<%{}> will be the default.
If this is not desired, you must dereference the argument yourself.

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.

=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

If the current package has a subroutine named C<ToLower>, it will be used to
change the case (See L<perlunicode/User-Defined Case Mappings>.)
Otherwise 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.  Any subroutine named
C<ToLower> will not be used.

=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 pragma, 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 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 link OLDFILE,NEWFILE
X<link>

Creates a new filename linked to the old filename.  Returns true for
success, false otherwise.

=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> is the month itself, 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<$year> is the number of years since 1900, not just the last two digits
of the year.  That is, C<$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 (to convert the second, minutes, hours, ... 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.

See L<perlport/localtime> for portability concerns.

The L<Time::gmtime> and L<Time::localtime> modules provides 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.

=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.

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 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 EXPR
X<lstat>

=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<$_>.

=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, @nums);

translates a list of numbers to the corresponding characters.  And

    %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<$!> (errno).
If omitted, MASK defaults to 0777. If omitted, FILENAME defaults
to C<$_>.

In general, it is better to create directories with 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">, C<IPC::SysV>, and C<IPC::Semaphore> documentation.

=item msgget KEY,FLAGS
X<msgget>

Calls the System V IPC function msgget(2).  Returns the message queue
id, or the undefined value if there is an error.  See also
L<perlipc/"SysV IPC"> and C<IPC::SysV> and C<IPC::Msg> documentation.

=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, or false if there is
an error.  See also L<perlipc/"SysV IPC">, C<IPC::SysV>, and
C<IPC::SysV::Msg> documentation.

=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, and be followed by the length of the actual message, and finally
the message itself.  This kind of packing can be achieved with
C<pack("l! a*", $type, $message)>.  Returns true if successful,
or false if there is an error.  See also C<IPC::SysV>
and C<IPC::SysV::Msg> documentation.

=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 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 $!;

and for writing:

    open(my $fh, '>', "output.txt") or die $!;

(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)
the variable is assigned a reference to a new anonymous filehandle,
otherwise if FILEHANDLE is an expression, its value is used as the name of
the real filehandle wanted.  (This is considered a symbolic reference, so
C<use strict 'refs'> should I<not> be in effect.)

If EXPR is omitted, the scalar variable of the same name as the
FILEHANDLE contains the filename.  (Note that lexical variables--those
declared with C<my>--will not work for this purpose; so if you're
using C<my>, specify EXPR in your call to open.)

If three or more arguments are specified then the mode of opening and
the filename are separate. If MODE is C<< '<' >> or nothing, the file
is opened for input.  If MODE is C<< '>' >>, the file is truncated and
opened for output, being created if necessary.  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 can't 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 two-argument (and one-argument) form of the call, the mode and
filename should be concatenated (in that order), possibly separated by
spaces.  You may omit the mode in these forms when that mode is
C<< '<' >>.

If the filename begins with C<'|'>, the filename is interpreted as a
command to which output is to be piped, and if the filename ends with a
C<'|'>, the filename is interpreted as a command that pipes output to
us.  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.)

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"> 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 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 UTF-8 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.

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 3-arg 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 "+<"
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 (i.e., 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 3-arg 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), and vice
versa.  But with C<< open(A, '>>&=B') >> the filehandles will share
the same file descriptor.

Note that if you are using Perls older than 5.8.0, Perl will be using
the standard C libraries' fdopen() to implement the "=" 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 Perl has been compiled with PerlIO or not by
running C<perl -V> and looking for 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<'-'>, i.e., either C<'|-'> or C<'-|'>
with 2-arguments (or 1-argument) form of open(), then
there is an implicit fork done, and the return value of open is the pid
of the child within the parent process, and C<0> within the child
process.  (Use C<defined($pid)> to determine whether the open was successful.)
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 shows the pipe as "list form", which is
not yet supported on all platforms.  A good rule of thumb is that if
your platform has true C<fork()> (in other words, if your platform is
Unix) you can use the list form.

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 $^F.  See L<perlvar/$^F>.

Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in C<$?> and
C<${^CHILD_ERROR_NATIVE}>.

The filename passed to 2-argument (or 1-argument) form of open() will
have leading and trailing whitespace deleted, and the 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 3-argument form to open a file with arbitrary weird characters in it,

    open(FOO, '<', $file);

otherwise it's necessary to protect any leading and trailing whitespace:

    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0");

(this may not work on some bizarre filesystems).  One should
conscientiously choose between the I<magic> and 3-arguments form
of open():

    open IN, $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];

will have exactly the opposite restrictions.

If you want a "real" C C<open> (see C<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 whatever variables hold references to
them, and automatically close whenever and however you leave that scope:

    use IO::File;
    #...
    sub read_myfile_munged {
        my $ALL = shift;
        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.
        $first;          # Or here.
    }

See L</seek> for some details about mixing reading and writing.

=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.
DIRHANDLEs have their own namespace separate from FILEHANDLEs.

See example at C<readdir>.

=item ord EXPR
X<ord> X<encoding>

=item ord

Returns the numeric (the native 8-bit encoding, like ASCII or EBCDIC,
or Unicode) value of the first character of EXPR.  If EXPR is an empty
string, returns 0.  If EXPR is omitted, uses C<$_>.

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>, which both 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 variable in the current package,
for use within the current scope.  In other words, C<our> has the same
scoping rules as C<my>, 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 character 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, dscribed 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 without any sort of trimming.

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
for 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.

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 the 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>.  Do not use any characters
but these 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:

    unpack("W/a", "\004Gurusamy")           gives ("Guru")
    unpack("a3/A A*", "007 Bond  J ")       gives (" Bond", "J")
    unpack("a3 x2 /A A*", "007: Bond, J.")  gives ("Bond, J", ".")

    pack("n/a* w/a","hello,","world")       gives "\000\006hello,\005world"
    pack("a/W2", ord("a") .. ord("z"))      gives "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 used/uses 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 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.

=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 that 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 VERSION
X<package> X<module> X<namespace> X<version>

=item package NAMESPACE

=item package NAMESPACE VERSION BLOCK
X<package> X<module> X<namespace> X<version>

=item package NAMESPACE BLOCK

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 the enclosing block, file, or eval (the same
as the C<my> operator).  All unqualified dynamic identifiers in this
scope will be in the given namespace, except where overridden by another
C<package> declaration.

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> (or C<our> (or C<state>)).  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">
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 pop ARRAY (or ARRAYREF)
X<pop> X<stack>

=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>.

If given a reference to an array, the argument will be dereferenced
automatically.

=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>.

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 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 standard output by default, or
to the last selected output channel; see L</select>.  If LIST is
also omitted, prints C<$_> to the currently selected output handle.
To set the default output handle to something other than STDOUT
use the select operation.  The current value of C<$,> (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, and any subroutine that you call will have one or more of
its expressions evaluated in list context.  Also 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 the arguments 
(or interpose a C<+>, but that doesn't look as good).

Note that if you're storing FILEHANDLEs in an array, or if you're using
any other expression more complex than a scalar variable to retrieve it,
you will have to use a block returning the filehandle value instead:

    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 FORMAT, LIST

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 C<sprintf>
for an explanation of the format argument.  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>.

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 (or ARRAYREF),LIST
X<push> X<stack>

Treats ARRAY as a stack, and pushes the values of LIST
onto 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>.

If given a reference to an array, the argument will be dereferenced
automatically.

=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 string input from
the user, quotemeta() or C<\Q> must be used.

=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.)

=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 C<sysread>.

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 UTF-8 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
*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<$!> (errno).  If EXPR is
omitted, uses C<$_>.

=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 UTF-8 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.

=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<$_> and returning 1, then returning 0 at
end of file.  If there is a filehandle, then the subroutine will be
called to act as a simple source filter, with the line as read in C<$_>.
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, blessed or unblessed); tied filehandles will be
ignored and return value 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 C<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.

=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 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 }>.

This keyword is available only when the "say" feature is
enabled: see L<feature>.

=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 for EXPR a
parenthesized list, 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 Fcntl module.  Returns C<1> on success, C<0>
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 will
default to this FILEHANDLE.  Second, references to variables related to
output will refer to this output channel.  For example, if you have 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);

=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) = split(' ',$_[0]);
        my($bits);
        for (@fhlist) {
            vec($bits,fileno($_),1) = 1;
        }
        $bits;
    }
    $rin = fhbits('STDIN TTY SOCK');

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 like select(2): it returns
-1 and sets C<$!>.

On some Unixes, select(2) may report a socket file
descriptor as "ready for reading" when no data is available, and
thus a subsequent read blocks. This can be avoided if you always use 
O_NONBLOCK on the socket. See select(2) and fcntl(2) for further
details.

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.

=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.

=item semget KEY,NSEMS,FLAGS
X<semget>

Calls the System V IPC function semget(2).  Returns the semaphore id, or
the undefined value if there is an error.  See also
L<perlipc/"SysV IPC">, C<IPC::SysV>, C<IPC::SysV::Semaphore>
documentation.

=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, or false if there is an 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.

=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()>.

=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).

=item setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
X<setsockopt>

Sets the socket option requested.  Returns undefined if there is an
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);

=item shift ARRAY (or ARRAYREF)
X<shift>

=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.

If given a reference to an array, the argument will be dereferenced
automatically.

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: the undefined value for error, "C<0> but
true" for zero, or the actual return value otherwise.
See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.

=item shmget KEY,SIZE,FLAGS
X<shmget>

Calls the System V IPC function shmget.  Returns the shared memory
segment id, or the undefined value if there is an error.
See also L<perlipc/"SysV IPC"> and C<IPC::SysV> documentation.

=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, or false if there is an error.
shmread() taints the variable. See also L<perlipc/"SysV IPC">,
C<IPC::SysV> documentation, and the C<IPC::Shareable> module from CPAN.

=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.

=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 counter-productive to declare $a and
$b as lexicals.

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, and 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), and because C<sort> raises an exception unless the
result of a comparison is defined, when sorting with a comparison function
like C<< $a <=> $b >>, be careful about 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 ARRAYREF),OFFSET,LENGTH,LIST
X<splice>

=item splice ARRAY (or ARRAYREF),OFFSET,LENGTH

=item splice ARRAY (or ARRAYREF),OFFSET

=item splice ARRAY (or ARRAYREF)

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.

If given a reference to an array, the argument will be dereferenced
automatically.

The following equivalences hold (assuming C<< $[ == 0 and $#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)) { ... }

=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 epmty 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 C<sprintf(3)> or C<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 'g' and '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 thoe 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:

   l           interpret integer as C type "long" or "unsigned long"
   h           interpret integer as C type "short" or "unsigned short"
   q, L or ll  interpret integer as C type "long long", "unsigned long long".
               or "quads" (typically 64-bit integers)

The last will raise an exception if Perl does not understand "quads" in your
installation. (This requires either that the platform natively support quads,
or that Perl were specifically compiled to support quads.) 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 other case is where you need a cryptographically-strong
starting point rather than the generally acceptable default, which is based on
time of day, process ID, and memory allocation, or the F</dev/urandom> device
if available.  And still 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.

For cryptographic purposes, however, you need something much more random 
than the default seed.  Checksumming the compressed output of one or more
rapidly changing operating system status programs is the usual method.  For
example:

    srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip -f`);

If you're particularly concerned with this, search the CPAN for
random number generator modules instead of rolling out your own.

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 exact results.

=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<$_>.  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 necessarily all are 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, 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.

=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> does.
However, those variables will never be reinitialized, contrary to
lexical variables that are reinitialized each time their enclosing block
is entered.

C<state> variables are enabled only when the C<use feature "state"> pragma 
is in effect.  See 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 on, and on 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 runs faster.  Those loops
that scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most.  You may have only
one C<study> active at a time: if you study a different scalar the first
is "unstudied".  (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, and does actually return
a value: the CODE ref of the closure you just created.

See L<perlsub> and L<perlref> for details about subroutines and
references, and L<attributes> and L<Attribute::Handlers> for more
information about attributes.

=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 C<0>, or whatever you've set C<$[> to (but don't do that).
If OFFSET is negative (or more precisely, less than C<$[>), starts
that far from the end of the string.  If LENGTH is omitted, returns
everything to 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 3-arg 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 };

=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<$!> (errno).
Note that some system calls can legitimately return C<-1>.  The proper
way to handle such calls is to assign C<$!=0;> before the call and
check the value of C<$!> if 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.  There is no way
to retrieve the file number of the other end.  You can avoid this
problem by using C<pipe> instead.

=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 name of the real filehandle wanted.  This function calls the
underlying operating system's C<open> function with the parameters
FILENAME, MODE, 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 C<open> 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.

=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 this 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.

=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 buffers 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 (UTF-8 encoded 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 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,
you may use either the L<Time::HiRes> module (from CPAN, and starting from
Perl 5.8 part of the standard distribution), 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 the 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.

=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, the undefined value
otherwise.

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> before writing to the file.

=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> (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.

=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 C<tie>.)
Has no effect if the variable is not tied.

=item unshift ARRAY (or ARRAYREF),LIST
X<unshift>

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.

If given a reference to an array, the argument will be dereferenced
automatically.

=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.

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> command 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 NUMERICAL 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.

=item values HASH (or HASHREF)
X<values>

=item values ARRAY (or ARRAYREF)

Returns a list consisting of all the values of the named hash, or the values
of an array. (In a 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

When given a reference to a hash or array, the argument will be
dereferenced automatically.

    for (values $hashref) { ... }
    for (values $obj->get_arrayref) { ... }

If the reference is a blessed object that overrides either C<%{}> or
C<@{}>, the override will be used instead of dereferencing the underlying
variable type.  If both overrides are provided, C<%{}> will be the default.
If this is not desired, you must dereference the argument yourself.

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.

=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.

=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 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, and then the record is written.
By default the top-of-page format is the name of the filehandle with
"_TOP" appended, but it may be dynamically set to the format of your
choice by assigning the name to the C<$^> variable while the filehandle is
selected.  The number of lines remaining on the current page is in
variable C<$->, 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