C<'> is taken to be a I<package qualifier>; see L<perlmod>.
Perl variable names may also be a sequence of digits or a single
-punctuation or control character. These names are all reserved for
+punctuation or control character (with the literal control character
+form deprecated). These names are all reserved for
special uses by Perl; for example, the all-digits names are used
to hold data captured by backreferences after a regular expression
match. Perl has a special syntax for the single-control-character
control-C<W>. This is better than typing a literal control-C<W>
into your program.
-Since Perl 5.6, Perl variable names may be alphanumeric
+Since Perl v5.6.0, Perl variable names may be alphanumeric
strings that begin with control characters (or better yet, a caret).
These variables must be written in the form C<${^Foo}>; the braces
are not optional. C<${^Foo}> denotes the scalar variable whose
also exempt from C<strict 'vars'> errors. A few other names are also
exempt in these ways:
- ENV STDIN
- INC STDOUT
- ARGV STDERR
- ARGVOUT _
- SIG
+ ENV STDIN
+ INC STDOUT
+ ARGV STDERR
+ ARGVOUT
+ SIG
In particular, the special C<${^_XYZ}> variables are always taken
to be in package C<main>, regardless of any C<package> declarations
=head1 SPECIAL VARIABLES
-The following names have special meaning to Perl. Most
-punctuation names have reasonable mnemonics, or analogs in the
-shells. Nevertheless, if you wish to use long variable names,
-you need only say
+The following names have special meaning to Perl. Most punctuation
+names have reasonable mnemonics, or analogs in the shells.
+Nevertheless, if you wish to use long variable names, you need only say:
use English;
-at the top of your program. This aliases all the short names to the long
-names in the current package. Some even have medium names, generally
-borrowed from B<awk>. In general, it's best to use the
+at the top of your program. This aliases all the short names to the long
+names in the current package. Some even have medium names, generally
+borrowed from B<awk>. For more info, please see L<English>.
- use English '-no_match_vars';
-
-invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids
-a certain performance hit with the use of regular expressions. See
-L<English>.
+Before you continue, note the sort order for variables. In general, we
+first list the variables in case-insensitive, almost-lexigraphical
+order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
+or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
+For variables with the same identifier, we list it in order of scalar,
+array, hash, and bareword.
=head2 General Variables
+=over 8
+
=item $ARG
=item $_
X<$_> X<$ARG>
-The default input and pattern-searching space. The following pairs are
+The default input and pattern-searching space. The following pairs are
equivalent:
- while (<>) {...} # equivalent only in while!
+ while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
chomp
chomp($_)
-Here are the places where Perl will assume C<$_> even if you
-don't use it:
+Here are the places where Perl will assume C<$_> even if you don't use it:
=over 3
=item *
-The following functions:
+The following functions use C<$_> as a default argument:
-abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob,
-hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
+abs, alarm, chomp, chop, chr, chroot,
+cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
+lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
-rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,
+rmdir, say, sin, split (for its second
+argument), sqrt, stat, study, uc, ucfirst,
unlink, unpack.
=item *
All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
See L<perlfunc/-X>
-
=item *
The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
=item *
-The default place to put an input record when a C<< <FH> >>
+The default place to put the next value or input record
+when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
operation's result is tested by itself as the sole criterion of a C<while>
-test. Outside a C<while> test, this will not happen.
+test. Outside a C<while> test, this will not happen.
=back
-As C<$_> is a global variable, this may lead in some cases to unwanted
-side-effects. As of perl 5.9.1, you can now use a lexical version of
-C<$_> by declaring it in a file or in a block with C<my>. Moreover,
-declaring C<our $_> restores the global C<$_> in the current scope.
+C<$_> is by default a global variable. However, as
+of perl v5.10.0, you can use a lexical version of
+C<$_> by declaring it in a file or in a block with C<my>. Moreover,
+declaring C<our $_> restores the global C<$_> in the current scope. Though
+this seemed like a good idea at the time it was introduced, lexical C<$_>
+actually causes more problems than it solves. If you call a function that
+expects to be passed information via C<$_>, it may or may not work,
+depending on how the function is written, there not being any easy way to
+solve this. Just avoid lexical C<$_>, unless you are feeling particularly
+masochistic. For this reason lexical C<$_> is still experimental and will
+produce a warning unless warnings have been disabled. As with other
+experimental features, the behavior of lexical C<$_> is subject to change
+without notice, including change into a fatal error.
Mnemonic: underline is understood in certain operations.
-=back
-
-=over 8
-
-=item $a
-
-=item $b
-X<$a> X<$b>
-
-Special package variables when using C<sort()>, see L<perlfunc/sort>.
-Because of this specialness C<$a> and C<$b> don't need to be declared
-(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
-pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
-be able to use them in the C<sort()> comparison block or function.
-
-=item $SUBSCRIPT_SEPARATOR
-
-=item $SUBSEP
-
-=item $;
-X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
-
-The subscript separator for multidimensional array emulation. If you
-refer to a hash element as
-
- $foo{$a,$b,$c}
-
-it really means
-
- $foo{join($;, $a, $b, $c)}
-
-But don't put
-
- @foo{$a,$b,$c} # a slice--note the @
-
-which means
-
- ($foo{$a},$foo{$b},$foo{$c})
+=item @ARG
-Default is "\034", the same as SUBSEP in B<awk>. If your
-keys contain binary data there might not be any safe value for C<$;>.
+=item @_
+X<@_> X<@ARG>
-Consider using "real" multidimensional arrays as described
-in L<perllol>.
+Within a subroutine the array C<@_> contains the parameters passed to
+that subroutine. Inside a subroutine, C<@_> is the default array for
+the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
-Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
+See L<perlsub>.
=item $LIST_SEPARATOR
When an array or an array slice is interpolated into a double-quoted
string or a similar context such as C</.../>, its elements are
-separated by this value. Default is a space. For example, this:
+separated by this value. Default is a space. For example, this:
print "The array is: @array\n";
Mnemonic: works in double-quoted context.
-=item ${^ENCODING}
-X<$^ENCODING>
-
-The I<object reference> to the C<Encode> object that is used to convert
-the source code to Unicode. Thanks to this variable your Perl script
-does not have to be written in UTF-8. Default is I<undef>. The direct
-manipulation of this variable is highly discouraged.
-
-This variable was added in Perl 5.8.2.
-
=item $PROCESS_ID
=item $PID
=item $$
X<$$> X<$PID> X<$PROCESS_ID>
-The process number of the Perl running this script. You should
-consider this variable read-only, although it will be altered
+The process number of the Perl running this script. Though you I<can> set
+this variable, doing so is generally discouraged, although it can be
+invaluable for some testing purposes. It will be reset automatically
across C<fork()> calls.
-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 C<$$>, whose value remains
-consistent across threads. If you want to call the underlying C<getpid()>,
-you may use the CPAN module C<Linux::Pid>.
+Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
+would emulate POSIX semantics on Linux systems using LinuxThreads, a
+partial implementation of POSIX Threads that has since been superseded
+by the Native POSIX Thread Library (NPTL).
-Mnemonic: same as shells.
-
-=item $REAL_USER_ID
+LinuxThreads is now obsolete on Linux, and caching C<getpid()>
+like this made embedding perl unnecessarily complex (since you'd have
+to manually update the value of $$), so now C<$$> and C<getppid()>
+will always return the same values as the underlying C library.
-=item $UID
+Debian GNU/kFreeBSD systems also used LinuxThreads up until and
+including the 6.0 release, but after that moved to FreeBSD thread
+semantics, which are POSIX-like.
-=item $<
-X<< $< >> X<$UID> X<$REAL_USER_ID>
+To see if your system is affected by this discrepancy check if
+C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
+value. NTPL threads preserve the POSIX semantics.
-The real uid of this process. You can change both the real uid and the
-effective uid at the same time by using C<POSIX::setuid()>. Since
-changes to C<< $< >> require a system call, check C<$!> after a change
-attempt to detect any possible errors.
+Mnemonic: same as shells.
-Mnemonic: it's the uid you came I<from>, if you're running setuid.
+=item $PROGRAM_NAME
-=item $EFFECTIVE_USER_ID
+=item $0
+X<$0> X<$PROGRAM_NAME>
-=item $EUID
+Contains the name of the program being executed.
-=item $>
-X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
+On some (but not all) operating systems assigning to C<$0> modifies
+the argument area that the C<ps> program sees. On some platforms you
+may have to use special C<ps> options or a different C<ps> to see the
+changes. Modifying the C<$0> is more useful as a way of indicating the
+current program state than it is for hiding the program you're
+running.
-The effective uid of this process. For example:
+Note that there are platform-specific limitations on the maximum
+length of C<$0>. In the most extreme case it may be limited to the
+space occupied by the original C<$0>.
- $< = $>; # set real to effective uid
- ($<,$>) = ($>,$<); # swap real and effective uids
+In some platforms there may be arbitrary amount of padding, for
+example space characters, after the modified name as shown by C<ps>.
+In some platforms this padding may extend all the way to the original
+length of the argument area, no matter what you do (this is the case
+for example with Linux 2.2).
-You can change both the effective uid and the real uid at the same
-time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
-to C<$!> to detect any possible errors after an attempted change.
+Note for BSD users: setting C<$0> does not completely remove "perl"
+from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
+result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
+and the " (perl)" suffix are shown depends on your exact BSD variant
+and version). This is an operating system feature, Perl cannot help it.
-C<< $< >> and C<< $> >> can be swapped only on machines
-supporting C<setreuid()>.
+In multithreaded scripts Perl coordinates the threads so that any
+thread may modify its copy of the C<$0> and the change becomes visible
+to ps(1) (assuming the operating system plays along). Note that
+the view of C<$0> the other threads have will not change since they
+have their own copies of it.
-Mnemonic: it's the uid you went I<to>, if you're running setuid.
+If the program has been given to perl via the switches C<-e> or C<-E>,
+C<$0> will contain the string C<"-e">.
+
+On Linux as of perl v5.14.0 the legacy process name will be set with
+C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
+perl has done since version 4.000. Now system utilities that read the
+legacy process name such as ps, top and killall will recognize the
+name you set when assigning to C<$0>. The string you supply will be
+cut off at 16 bytes, this is a limitation imposed by Linux.
+
+Mnemonic: same as B<sh> and B<ksh>.
=item $REAL_GROUP_ID
=item $(
X<$(> X<$GID> X<$REAL_GROUP_ID>
-The real gid of this process. If you are on a machine that supports
+The real gid of this process. If you are on a machine that supports
membership in multiple groups simultaneously, gives a space separated
-list of groups you are in. The first number is the one returned by
+list of groups you are in. The first number is the one returned by
C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
the same as the first number.
However, a value assigned to C<$(> must be a single number used to
-set the real gid. So the value given by C<$(> should I<not> be assigned
-back to C<$(> without being forced numeric, such as by adding zero. Note
+set the real gid. So the value given by C<$(> should I<not> be assigned
+back to C<$(> without being forced numeric, such as by adding zero. Note
that this is different to the effective gid (C<$)>) which does take a
list.
You can change both the real gid and the effective gid at the same
-time by using C<POSIX::setgid()>. Changes to C<$(> require a check to C<$!>
+time by using C<POSIX::setgid()>. Changes
+to C<$(> require a check to C<$!>
to detect any possible errors after an attempted change.
-Mnemonic: parentheses are used to I<group> things. The real gid is the
+Mnemonic: parentheses are used to I<group> things. The real gid is the
group you I<left>, if you're running setgid.
=item $EFFECTIVE_GROUP_ID
=item $)
X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
-The effective gid of this process. If you are on a machine that
+The effective gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a space
-separated list of groups you are in. The first number is the one
+separated list of groups you are in. The first number is the one
returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
one of which may be the same as the first number.
Similarly, a value assigned to C<$)> must also be a space-separated
-list of numbers. The first number sets the effective gid, and
-the rest (if any) are passed to C<setgroups()>. To get the effect of an
+list of numbers. The first number sets the effective gid, and
+the rest (if any) are passed to C<setgroups()>. To get the effect of an
empty list for C<setgroups()>, just repeat the new effective gid; that is,
to force an effective gid of 5 and an effectively empty C<setgroups()>
list, say C< $) = "5 5" >.
after an attempted change.
C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
-machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
+machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
and C<$)> can be swapped only on machines supporting C<setregid()>.
-Mnemonic: parentheses are used to I<group> things. The effective gid
+Mnemonic: parentheses are used to I<group> things. The effective gid
is the group that's I<right> for you, if you're running setgid.
-=item $PROGRAM_NAME
+=item $REAL_USER_ID
-=item $0
-X<$0> X<$PROGRAM_NAME>
+=item $UID
-Contains the name of the program being executed.
+=item $<
+X<< $< >> X<$UID> X<$REAL_USER_ID>
-On some (but not all) operating systems assigning to C<$0> modifies
-the argument area that the C<ps> program sees. On some platforms you
-may have to use special C<ps> options or a different C<ps> to see the
-changes. Modifying the C<$0> is more useful as a way of indicating the
-current program state than it is for hiding the program you're
-running.
+The real uid of this process. You can change both the real uid and the
+effective uid at the same time by using C<POSIX::setuid()>. Since
+changes to C<< $< >> require a system call, check C<$!> after a change
+attempt to detect any possible errors.
-Note that there are platform-specific limitations on the maximum
-length of C<$0>. In the most extreme case it may be limited to the
-space occupied by the original C<$0>.
+Mnemonic: it's the uid you came I<from>, if you're running setuid.
-In some platforms there may be arbitrary amount of padding, for
-example space characters, after the modified name as shown by C<ps>.
-In some platforms this padding may extend all the way to the original
-length of the argument area, no matter what you do (this is the case
-for example with Linux 2.2).
+=item $EFFECTIVE_USER_ID
-Note for BSD users: setting C<$0> does not completely remove "perl"
-from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
-result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
-and the " (perl)" suffix are shown depends on your exact BSD variant
-and version). This is an operating system feature, Perl cannot help it.
+=item $EUID
-In multithreaded scripts Perl coordinates the threads so that any
-thread may modify its copy of the C<$0> and the change becomes visible
-to ps(1) (assuming the operating system plays along). Note that
-the view of C<$0> the other threads have will not change since they
-have their own copies of it.
+=item $>
+X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
-If the program has been given to perl via the switches C<-e> or C<-E>,
-C<$0> will contain the string C<"-e">.
+The effective uid of this process. For example:
-On Linux as of perl 5.14 the legacy process name will be set with
-L<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
-perl has done since version 4.000. Now system utilities that read the
-legacy process name such as ps, top and killall will recognize the
-name you set when assigning to C<$0>. The string you supply will be
-cut off at 16 bytes, this is a limitation imposed by Linux.
+ $< = $>; # set real to effective uid
+ ($<,$>) = ($>,$<); # swap real and effective uids
-Mnemonic: same as B<sh> and B<ksh>.
-
-=item $COMPILING
+You can change both the effective uid and the real uid at the same
+time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
+to C<$!> to detect any possible errors after an attempted change.
-=item $^C
-X<$^C> X<$COMPILING>
+C<< $< >> and C<< $> >> can be swapped only on machines
+supporting C<setreuid()>.
-The current value of the flag associated with the B<-c> switch.
-Mainly of use with B<-MO=...> to allow code to alter its behavior
-when being compiled, such as for example to C<AUTOLOAD> at compile
-time rather than normal, deferred loading. Setting
-C<$^C = 1> is similar to calling C<B::minus_c>.
+Mnemonic: it's the uid you went I<to>, if you're running setuid.
-This variable was added in Perl 5.6.
+=item $SUBSCRIPT_SEPARATOR
-=item $DEBUGGING
+=item $SUBSEP
-=item $^D
-X<$^D> X<$DEBUGGING>
+=item $;
+X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
-The current value of the debugging flags. May be read or set. Like its
-command-line equivalent, you can use numeric or symbolic values, eg
-C<$^D = 10> or C<$^D = "st">.
+The subscript separator for multidimensional array emulation. If you
+refer to a hash element as
-Mnemonic: value of B<-D> switch.
+ $foo{$x,$y,$z}
-=item $SYSTEM_FD_MAX
+it really means
-=item $^F
-X<$^F> X<$SYSTEM_FD_MAX>
+ $foo{join($;, $x, $y, $z)}
-The maximum system file descriptor, ordinarily 2. System file
-descriptors are passed to C<exec()>ed processes, while higher file
-descriptors are not. Also, during an C<open()>, system file descriptors are
-preserved even if the C<open()> fails (ordinary file descriptors are
-closed before the C<open()> is attempted). The close-on-exec
-status of a file descriptor will be decided according to the value of
-C<$^F> when the corresponding file, pipe, or socket was opened, not the
-time of the C<exec()>.
+But don't put
-=item $^H
+ @foo{$x,$y,$z} # a slice--note the @
-WARNING: This variable is strictly for internal use only. Its availability,
-behavior, and contents are subject to change without notice.
+which means
-This variable contains compile-time hints for the Perl interpreter. At the
-end of compilation of a BLOCK the value of this variable is restored to the
-value when the interpreter started to compile the BLOCK.
+ ($foo{$x},$foo{$y},$foo{$z})
-When perl begins to parse any block construct that provides a lexical scope
-(e.g., eval body, required file, subroutine body, loop body, or conditional
-block), the existing value of C<$^H> is saved, but its value is left unchanged.
-When the compilation of the block is completed, it regains the saved value.
-Between the points where its value is saved and restored, code that
-executes within BEGIN blocks is free to change the value of C<$^H>.
+Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
+binary data there might not be any safe value for C<$;>.
-This behavior provides the semantic of lexical scoping, and is used in,
-for instance, the C<use strict> pragma.
+Consider using "real" multidimensional arrays as described
+in L<perllol>.
-The contents should be an integer; different bits of it are used for
-different pragmatic flags. Here's an example:
+Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
- sub add_100 { $^H |= 0x100 }
+=item $a
- sub foo {
- BEGIN { add_100() }
- bar->baz($boon);
- }
+=item $b
+X<$a> X<$b>
-Consider what happens during execution of the BEGIN block. At this point
-the BEGIN block has already been compiled, but the body of C<foo()> is still
-being compiled. The new value of C<$^H> will therefore be visible only while
-the body of C<foo()> is being compiled.
+Special package variables when using C<sort()>, see L<perlfunc/sort>.
+Because of this specialness C<$a> and C<$b> don't need to be declared
+(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
+pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
+be able to use them in the C<sort()> comparison block or function.
-Substitution of the above BEGIN block with:
+=item %ENV
+X<%ENV>
- BEGIN { require strict; strict->import('vars') }
+The hash C<%ENV> contains your current environment. Setting a
+value in C<ENV> changes the environment for any child processes
+you subsequently C<fork()> off.
-demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
-version of the same lexical pragma:
+As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
- BEGIN { require strict; strict->import('vars') if $condition }
+ my $foo = 1;
+ $ENV{'bar'} = \$foo;
+ if( ref $ENV{'bar'} ) {
+ say "Pre 5.18.0 Behaviour";
+ } else {
+ say "Post 5.18.0 Behaviour";
+ }
-This variable was added in Perl 5.003.
+Previously, only child processes received stringified values:
-=item %^H
+ my $foo = 1;
+ $ENV{'bar'} = \$foo;
-The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
-useful for implementation of lexically scoped pragmas. See L<perlpragma>.
+ # Always printed 'non ref'
+ system($^X, '-e',
+ q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
-This variable was added in Perl 5.6.
+This happens because you can't really share arbitrary data structures with
+foreign processes.
-=item $INPLACE_EDIT
+=item $SYSTEM_FD_MAX
-=item $^I
-X<$^I> X<$INPLACE_EDIT>
+=item $^F
+X<$^F> X<$SYSTEM_FD_MAX>
-The current value of the inplace-edit extension. Use C<undef> to disable
-inplace editing.
+The maximum system file descriptor, ordinarily 2. System file
+descriptors are passed to C<exec()>ed processes, while higher file
+descriptors are not. Also, during an
+C<open()>, system file descriptors are
+preserved even if the C<open()> fails (ordinary file descriptors are
+closed before the C<open()> is attempted). The close-on-exec
+status of a file descriptor will be decided according to the value of
+C<$^F> when the corresponding file, pipe, or socket was opened, not the
+time of the C<exec()>.
-Mnemonic: value of B<-i> switch.
+=item @F
+X<@F>
-=item $^M
-X<$^M>
+The array C<@F> contains the fields of each line read in when autosplit
+mode is turned on. See L<perlrun> for the B<-a> switch. This array
+is package-specific, and must be declared or given a full package name
+if not in package main when running under C<strict 'vars'>.
-By default, running out of memory is an untrappable, fatal error.
-However, if suitably built, Perl can use the contents of C<$^M>
-as an emergency memory pool after C<die()>ing. Suppose that your Perl
-were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
-Then
+=item @INC
+X<@INC>
- $^M = 'a' x (1 << 16);
+The array C<@INC> contains the list of places that the C<do EXPR>,
+C<require>, or C<use> constructs look for their library files. It
+initially consists of the arguments to any B<-I> command-line
+switches, followed by the default Perl library, probably
+F</usr/local/lib/perl>, followed by ".", to represent the current
+directory. ("." will not be appended if taint checks are enabled,
+either by C<-T> or by C<-t>.) If you need to modify this at runtime,
+you should use the C<use lib> pragma to get the machine-dependent
+library properly loaded also:
-would allocate a 64K buffer for use in an emergency. See the
-F<INSTALL> file in the Perl distribution for information on how to
-add custom C compilation flags when compiling perl. To discourage casual
-use of this advanced feature, there is no L<English|English> long name for
-this variable.
+ use lib '/mypath/libdir/';
+ use SomeMod;
-This variable was added in Perl 5.004.
-
-=item $OSNAME
-
-=item $^O
-X<$^O> X<$OSNAME>
-
-The name of the operating system under which this copy of Perl was
-built, as determined during the configuration process. For examples
-see L<perlport/PLATFORMS>.
-
-The value is identical to C<$Config{'osname'}>. See also L<Config>
-and the B<-V> command-line switch documented in L<perlrun>.
-
-In Windows platforms, C<$^O> is not very helpful: since it is always
-C<MSWin32>, it doesn't tell the difference between
-95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
-Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
-between the variants.
-
-This variable was added in Perl 5.003.
-
-=item ${^OPEN}
-
-An internal variable used by PerlIO. A string in two parts, separated
-by a C<\0> byte, the first part describes the input layers, the second
-part describes the output layers.
-
-This variable was added in Perl 5.8.2.
-
-=item $PERLDB
-
-=item $^P
-X<$^P> X<$PERLDB>
-
-The internal variable for debugging support. The meanings of the
-various bits are subject to change, but currently indicate:
-
-=over 6
-
-=item 0x01
-
-Debug subroutine enter/exit.
-
-=item 0x02
-
-Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
-statement executed. Also causes saving source code lines (like 0x400).
-
-=item 0x04
-
-Switch off optimizations.
-
-=item 0x08
-
-Preserve more data for future interactive inspections.
-
-=item 0x10
-
-Keep info about source lines on which a subroutine is defined.
-
-=item 0x20
-
-Start with single-step on.
-
-=item 0x40
-
-Use subroutine address instead of name when reporting.
-
-=item 0x80
-
-Report C<goto &subroutine> as well.
-
-=item 0x100
-
-Provide informative "file" names for evals based on the place they were compiled.
-
-=item 0x200
-
-Provide informative names to anonymous subroutines based on the place they
-were compiled.
-
-=item 0x400
+You can also insert hooks into the file inclusion system by putting Perl
+code directly into C<@INC>. Those hooks may be subroutine references,
+array references or blessed objects. See L<perlfunc/require> for details.
-Save source code lines into C<@{"_<$filename"}>.
+=item %INC
+X<%INC>
-=back
+The hash C<%INC> contains entries for each filename included via the
+C<do>, C<require>, or C<use> operators. The key is the filename
+you specified (with module names converted to pathnames), and the
+value is the location of the file found. The C<require>
+operator uses this hash to determine whether a particular file has
+already been included.
-Some bits may be relevant at compile-time only, some at
-run-time only. This is a new mechanism and the details may change.
-See also L<perldebguts>.
+If the file was loaded via a hook (e.g. a subroutine reference, see
+L<perlfunc/require> for a description of these hooks), this hook is
+by default inserted into C<%INC> in place of a filename. Note, however,
+that the hook may have set the C<%INC> entry by itself to provide some more
+specific info.
-=item @F
-X<@F>
+=item $INPLACE_EDIT
-The array C<@F> contains the fields of each line read in when autosplit
-mode is turned on. See L<perlrun> for the B<-a> switch. This array
-is package-specific, and must be declared or given a full package name
-if not in package main when running under C<strict 'vars'>.
+=item $^I
+X<$^I> X<$INPLACE_EDIT>
-=item @INC
-X<@INC>
+The current value of the inplace-edit extension. Use C<undef> to disable
+inplace editing.
-The array C<@INC> contains the list of places that the C<do EXPR>,
-C<require>, or C<use> constructs look for their library files. It
-initially consists of the arguments to any B<-I> command-line
-switches, followed by the default Perl library, probably
-F</usr/local/lib/perl>, followed by ".", to represent the current
-directory. ("." will not be appended if taint checks are enabled,
-either by C<-T> or by C<-t>.) If you need to modify this at runtime,
-you should use the C<use lib> pragma to get the machine-dependent
-library properly loaded also:
+Mnemonic: value of B<-i> switch.
- use lib '/mypath/libdir/';
- use SomeMod;
+=item $^M
+X<$^M>
-You can also insert hooks into the file inclusion system by putting Perl
-code directly into C<@INC>. Those hooks may be subroutine references, array
-references or blessed objects. See L<perlfunc/require> for details.
+By default, running out of memory is an untrappable, fatal error.
+However, if suitably built, Perl can use the contents of C<$^M>
+as an emergency memory pool after C<die()>ing. Suppose that your Perl
+were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
+Then
-=item @ARG
+ $^M = 'a' x (1 << 16);
-=item @_
-X<@_> X<@ARG>
+would allocate a 64K buffer for use in an emergency. See the
+F<INSTALL> file in the Perl distribution for information on how to
+add custom C compilation flags when compiling perl. To discourage casual
+use of this advanced feature, there is no L<English|English> long name for
+this variable.
-Within a subroutine the array C<@_> contains the parameters passed to that
-subroutine. See L<perlsub>.
+This variable was added in Perl 5.004.
-=item %INC
-X<%INC>
+=item $OSNAME
-The hash C<%INC> contains entries for each filename included via the
-C<do>, C<require>, or C<use> operators. The key is the filename
-you specified (with module names converted to pathnames), and the
-value is the location of the file found. The C<require>
-operator uses this hash to determine whether a particular file has
-already been included.
+=item $^O
+X<$^O> X<$OSNAME>
-If the file was loaded via a hook (e.g. a subroutine reference, see
-L<perlfunc/require> for a description of these hooks), this hook is
-by default inserted into C<%INC> in place of a filename. Note, however,
-that the hook may have set the C<%INC> entry by itself to provide some more
-specific info.
+The name of the operating system under which this copy of Perl was
+built, as determined during the configuration process. For examples
+see L<perlport/PLATFORMS>.
-=item %ENV
+The value is identical to C<$Config{'osname'}>. See also L<Config>
+and the B<-V> command-line switch documented in L<perlrun>.
-=item $ENV{expr}
-X<%ENV>
+In Windows platforms, C<$^O> is not very helpful: since it is always
+C<MSWin32>, it doesn't tell the difference between
+95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
+Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
+between the variants.
-The hash C<%ENV> contains your current environment. Setting a
-value in C<ENV> changes the environment for any child processes
-you subsequently C<fork()> off.
+This variable was added in Perl 5.003.
=item %SIG
-
-=item $SIG{expr}
X<%SIG>
-The hash C<%SIG> contains signal handlers for signals. For example:
+The hash C<%SIG> contains signal handlers for signals. For example:
- sub handler { # 1st argument is signal name
+ sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
- }
+ }
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
- $SIG{'INT'} = 'DEFAULT'; # restore default action
- $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
+ $SIG{'INT'} = 'DEFAULT'; # restore default action
+ $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
Using a value of C<'IGNORE'> usually has the effect of ignoring the
-signal, except for the C<CHLD> signal. See L<perlipc> for more about
+signal, except for the C<CHLD> signal.
See L<perlipc> for more about
this special case.
Here are some other examples:
- $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
- $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
+ $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
+ # recommended)
+ $SIG{"PIPE"} = \&Plumber; # just fine; assume current
+ # Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
- $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
+ $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
+ # return??
Be sure not to use a bareword as the name of a signal handler,
lest you inadvertently call it.
If your system has the C<sigaction()> function then signal handlers
-are installed using it. This means you get reliable signal handling.
+are installed using it. This means you get reliable signal handling.
-The default delivery policy of signals changed in Perl 5.8.0 from
+The default delivery policy of signals changed in Perl v5.8.0 from
immediate (also known as "unsafe") to deferred, also known as "safe
signals". See L<perlipc> for more information.
-Certain internal hooks can be also set using the C<%SIG> hash. The
+Certain internal hooks can be also set using the C<%SIG> hash. The
routine indicated by C<$SIG{__WARN__}> is called when a warning
message is about to be printed. The warning message is passed as the
first argument. The presence of a C<__WARN__> hook causes the
-ordinary printing of warnings to C<STDERR> to be suppressed. You can
+ordinary printing of warnings to C<STDERR> to be suppressed. You can
use this to save warnings in a variable, or turn warnings into fatal
errors, like this:
local $SIG{__WARN__} = sub {};
The routine indicated by C<$SIG{__DIE__}> is called when a fatal
-exception is about to be thrown. The error message is passed as the
-first argument. When a C<__DIE__> hook routine returns, the exception
+exception is about to be thrown. The error message is passed as the
+first argument. When a C<__DIE__> hook routine returns, the exception
processing continues as it would have in the absence of the hook,
-unless the hook routine itself exits via a C<goto>, a loop exit, or a
-C<die()>. The C<__DIE__> handler is explicitly disabled during the
-call, so that you can die from a C<__DIE__> handler. Similarly for
-C<__WARN__>.
+unless the hook routine itself exits via a C<goto &sub>, a loop exit,
+or a C<die()>. The C<__DIE__> handler is explicitly disabled during
+the call, so that you can die from a C<__DIE__> handler. Similarly
+for C<__WARN__>.
Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
-even inside an C<eval()>. Do not use this to rewrite a pending
+even inside an C<eval()>. Do not use this to rewrite a pending
exception in C<$@>, or as a bizarre substitute for overriding
-C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
+C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
in a future release so that C<$SIG{__DIE__}> is only called if your
-program is about to exit, as was the original intent. Any other use is
+program is about to exit, as was the original intent. Any other use is
deprecated.
C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
-may be called to report (probable) errors found by the parser. In such
+may be called to report (probable) errors found by the parser. In such
a case the parser may be in inconsistent state, so any attempt to
evaluate Perl code from such a handler will probably result in a
-segfault. This means that warnings or errors that result from parsing
+segfault. This means that warnings or errors that result from parsing
Perl should be used with extreme caution, like this:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
- die "Something wrong, but could not load Carp to give backtrace...
- To see backtrace try starting Perl with -MCarp switch";
+ die "Something wrong, but could not load Carp to give "
+ . "backtrace...\n\t"
+ . "To see backtrace try starting Perl with -MCarp switch";
Here the first line will load C<Carp> I<unless> it is the parser who
-called the handler. The second line will print backtrace and die if
-C<Carp> was available. The third line will be executed only if C<Carp> was
+called the handler. The second line will print backtrace and die if
+C<Carp> was available. The third line will be executed only if C<Carp> was
not available.
+Having to even think about the C<$^S> variable in your exception
+handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
+invites grievous and difficult to track down errors. Avoid it
+and use an C<END{}> or CORE::GLOBAL::die override instead.
+
See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
L<warnings> for additional information.
-=back
-
-=head2 Names that are no longer special
-
-These variables had special meaning in prior versions of Perl but now
-have no effect and will cause warnings if used. They are included
-here for historical reference.
-
-=over 8
-
=item $BASETIME
=item $^T
X<$^T> X<$BASETIME>
The time at which the program began running, in seconds since the
-epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
+epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
and B<-C> filetests are based on this value.
-=item ${^TAINT}
-
-Reflects if taint mode is on or off. 1 for on (the program was run with
-B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
-B<-t> or B<-TU>).
-
-This variable is read-only.
-
-This variable was added in Perl 5.8.
-
-=item ${^UNICODE}
-
-Reflects certain Unicode settings of Perl. See L<perlrun>
-documentation for the C<-C> switch for more information about
-the possible values.
-
-This variable is set during Perl startup and is thereafter read-only.
-
-This variable was added in Perl 5.8.2.
-
-=item ${^UTF8CACHE}
-
-This variable controls the state of the internal UTF-8 offset caching code.
-1 for on (the default), 0 for off, -1 to debug the caching code by checking
-all its results against linear scans, and panicking on any discrepancy.
-
-This variable was added in Perl 5.8.9.
-
-=item ${^UTF8LOCALE}
-
-This variable indicates whether a UTF-8 locale was detected by perl at
-startup. This information is used by perl when it's in
-adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
-switch); see L<perlrun> for more info on this.
-
-This variable was added in Perl 5.8.8.
-
=item $PERL_VERSION
=item $^V
The revision, version, and subversion of the Perl interpreter,
represented as a C<version> object.
-This variable first appeared in perl 5.6.0; earlier versions of perl
-will see an undefined value. Before perl 5.10.0 C<$^V> was represented
+This variable first appeared in perl v5.6.0; earlier versions of perl
+will see an undefined value. Before perl v5.10.0 C<$^V> was represented
as a v-string.
C<$^V> can be used to determine whether the Perl interpreter executing
-a script is in the right range of versions. For example:
+a script is in the right range of versions. For example:
warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
See also C<$]> for an older representation of the Perl version.
-This variable was added in Perl 5.6.
+This variable was added in Perl v5.6.0.
Mnemonic: use ^V for Version Control.
=item ${^WIN32_SLOPPY_STAT}
-X<sitecustomize> X<sitecustomize.pl>
+X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
If this variable is set to a true value, then C<stat()> on Windows will
-not try to open the file. This means that the link count cannot be
+not try to open the file. This means that the link count cannot be
determined and file attributes may be out of date if additional
-hardlinks to the file exist. On the other hand, not opening the file
+hardlinks to the file exist. On the other hand, not opening the file
is considerably faster, especially for files on network drives.
This variable could be set in the F<sitecustomize.pl> file to
configure the local Perl installation to use "sloppy" C<stat()> by
-default. See the documentation for B<-f> in
+default. See the documentation for B<-f> in
L<perlrun|perlrun/"Command Switches"> for more information about site
customization.
-This variable was added in Perl 5.10.
+This variable was added in Perl v5.10.0.
=item $EXECUTABLE_NAME
Depending on the host operating system, the value of C<$^X> may be
a relative or absolute pathname of the perl program file, or may
be the string used to invoke perl but not the pathname of the
-perl program file. Also, most operating systems permit invoking
+perl program file. Also, most operating systems permit invoking
programs that are not in the PATH environment variable, so there
-is no guarantee that the value of C<$^X> is in PATH. For VMS, the
+is no guarantee that the value of C<$^X> is in PATH. For VMS, the
value may or may not include a version number.
You usually can use the value of C<$^X> to re-invoke an independent
copy of the same perl that is currently running, e.g.,
- @first_run = `$^X -le "print int rand 100 for 1..100"`;
+ @first_run = `$^X -le "print int rand 100 for 1..100"`;
But recall that not all operating systems support forking or
capturing of the output of commands, so this complex statement
It is not safe to use the value of C<$^X> as a path name of a file,
as some operating systems that have a mandatory suffix on
executable files do not require use of the suffix when invoking
-a command. To convert the value of C<$^X> to a path name, use the
+a command. To convert the value of C<$^X> to a path name, use the
following statements:
- # Build up a set of file names (not command names).
- use Config;
- $this_perl = $^X;
- if ($^O ne 'VMS')
- {$this_perl .= $Config{_exe}
- unless $this_perl =~ m/$Config{_exe}$/i;}
+ # Build up a set of file names (not command names).
+ use Config;
+ my $this_perl = $^X;
+ if ($^O ne 'VMS') {
+ $this_perl .= $Config{_exe}
+ unless $this_perl =~ m/$Config{_exe}$/i;
+ }
Because many operating systems permit anyone with read access to
the Perl program file to make a copy of it, patch the copy, and
then execute the copy, the security-conscious Perl programmer
should take care to invoke the installed copy of perl, not the
-copy referenced by C<$^X>. The following statements accomplish
+copy referenced by C<$^X>. The following statements accomplish
this goal, and produce a pathname that can be invoked as a
command or referenced as a file.
- use Config;
- $secure_perl_path = $Config{perlpath};
- if ($^O ne 'VMS')
- {$secure_perl_path .= $Config{_exe}
- unless $secure_perl_path =~ m/$Config{_exe}$/i;}
+ use Config;
+ my $secure_perl_path = $Config{perlpath};
+ if ($^O ne 'VMS') {
+ $secure_perl_path .= $Config{_exe}
+ unless $secure_perl_path =~ m/$Config{_exe}$/i;
+ }
=back
=head2 Variables related to regular expressions
Most of the special variables related to regular expressions are side
-effects. Perl sets these variables when it has a successful match, so
-you should check the match result before using them. For instance:
+effects. Perl sets these variables when it has a successful match, so
+you should check the match result before using them. For instance:
- if( /P(A)TT(ER)N/ ) {
- print "I found $1 and $2\n";
- }
+ if( /P(A)TT(ER)N/ ) {
+ print "I found $1 and $2\n";
+ }
-These variables are read-only and dynamically-scoped, unless we note
+These variables are read-only and dynamically-scoped, unless we note
otherwise.
-The dynamic nature of the regular expression variables means that their value
-is limited to the block that they are in, as demonstrated by this bit of code:
-
- my $outer = 'Wallace and Grommit';
- my $inner = 'Mutt and Jeff';
-
- my $pattern = qr/(\S+) and (\S+)/;
-
- sub show_n { print "\$1 is $1; \$2 is $2\n" }
-
- {
- OUTER:
- show_n() if $outer =~ m/$pattern/;
-
- INNER: {
- show_n() if $inner =~ m/$pattern/;
- }
-
- show_n();
- }
+The dynamic nature of the regular expression variables means that
+their value is limited to the block that they are in, as demonstrated
+by this bit of code:
+
+ my $outer = 'Wallace and Grommit';
+ my $inner = 'Mutt and Jeff';
+
+ my $pattern = qr/(\S+) and (\S+)/;
+
+ sub show_n { print "\$1 is $1; \$2 is $2\n" }
+
+ {
+ OUTER:
+ show_n() if $outer =~ m/$pattern/;
+
+ INNER: {
+ show_n() if $inner =~ m/$pattern/;
+ }
-The output shows that while in the C<OUTER> block, the values of C<$1> and C<$2>
-are from the match against C<$outer>. Inside the C<INNER> block, the values of
-C<$1> and C<$2> are from the match against C<$inner>, but only until the end of the
-block (i.e. the dynamic scope). After the C<INNER> block completes, the values of
-C<$1> and C<$2> return to the values for the match against C<$outer> even though
+ show_n();
+ }
+
+The output shows that while in the C<OUTER> block, the values of C<$1>
+and C<$2> are from the match against C<$outer>. Inside the C<INNER>
+block, the values of C<$1> and C<$2> are from the match against
+C<$inner>, but only until the end of the block (i.e. the dynamic
+scope). After the C<INNER> block completes, the values of C<$1> and
+C<$2> return to the values for the match against C<$outer> even though
we have not made another match:
- $1 is Wallace; $2 is Grommit
- $1 is Mutt; $2 is Jeff
- $1 is Wallace; $2 is Grommit
+ $1 is Wallace; $2 is Grommit
+ $1 is Mutt; $2 is Jeff
+ $1 is Wallace; $2 is Grommit
+
+=head3 Performance issues
+
+Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
+or C<$'> (or their C<use English> equivalents) anywhere in the code, caused
+all subsequent successful pattern matches to make a copy of the matched
+string, in case the code might subsequently access one of those variables.
+This imposed a considerable performance penalty across the whole program,
+so generally the use of these variables has been discouraged.
+
+In Perl 5.6.0 the C<@-> and C<@+> dynamic arrays were introduced that
+supply the indices of successful matches. So you could for example do
+this:
+
+ $str =~ /pattern/;
+
+ print $`, $&, $'; # bad: perfomance hit
+
+ print # good: no perfomance hit
+ substr($str, 0, $-[0]),
+ substr($str, $-[0], $+[0]-$-[0]),
+ substr($str, $+[0]);
+
+In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
+C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
+you to suffer the penalties only on patterns marked with C</p>.
+
+In Perl 5.18.0 onwards, perl started noting the presence of each of the
+three variables separately, and only copied that part of the string
+required; so in
+
+ $`; $&; "abcdefgh" =~ /d/
+
+perl would only copy the "abcd" part of the string. That could make a big
+difference in something like
+
+ $str = 'x' x 1_000_000;
+ $&; # whoops
+ $str =~ /x/g # one char copied a million times, not a million chars
+
+In Perl 5.20.0 a new copy-on-write system was enabled by default, which
+finally fixes all performance issues with these three variables, and makes
+them safe to use anywhere.
+
+The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
+find uses of these problematic match variables in your code.
=over 8
any matches hidden within a BLOCK or C<eval()> enclosed by the current
BLOCK).
-The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See L</BUGS>.
-To avoid this penatly, you can extract the same substring by
-using L</@->. Starting with Perl 5.10, you can use the </p> match flag
-and the C<${^MATCH}> variable to do the same thing for particular
-match operations.
+See L</Performance issues> above for the serious performance implications
+of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: like C<&> in some editors.
-
+
=item ${^MATCH}
X<${^MATCH}>
This is similar to C<$&> (C<$MATCH>) except that it does not incur the
-performance penalty associated with that variable, and is only guaranteed
+performance penalty associated with that variable.
+
+See L</Performance issues> above.
+
+In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
-the C</p> modifier.
+the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
+C<${^MATCH}> does the same thing as C<$MATCH>.
-This variable was added in Perl 5.10.
+This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
=item $PREMATCH
=item $`
-X<$`> X<$PREMATCH>
+X<$`> X<$PREMATCH> X<${^PREMATCH}>
The string preceding whatever was matched by the last successful
pattern match, not counting any matches hidden within a BLOCK or C<eval>
-enclosed by the current BLOCK.
+enclosed by the current BLOCK.
-The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See L</BUGS>.
-To avoid this penatly, you can extract the same substring by
-using L</@->. Starting with Perl 5.10, you can use the </p> match flag
-and the C<${^PREMATCH}> variable to do the same thing for particular
-match operations.
+See L</Performance issues> above for the serious performance implications
+of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: C<`> often precedes a quoted string.
=item ${^PREMATCH}
-X<${^PREMATCH}>
+X<$`> X<${^PREMATCH}>
This is similar to C<$`> ($PREMATCH) except that it does not incur the
-performance penalty associated with that variable, and is only guaranteed
+performance penalty associated with that variable.
+
+See L</Performance issues> above.
+
+In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
-the C</p> modifier.
+the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
+C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
-This variable was added in Perl 5.10
+This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
=item $POSTMATCH
=item $'
-X<$'> X<$POSTMATCH>
+X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or C<eval()>
-enclosed by the current BLOCK). Example:
+enclosed by the current BLOCK). Example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
-The use of this variable anywhere in a program imposes a considerable
-performance penalty on all regular expression matches. See L</BUGS>.
-To avoid this penatly, you can extract the same substring by
-using L</@->. Starting with Perl 5.10, you can use the </p> match flag
-and the C<${^POSTMATCH}> variable to do the same thing for particular
-match operations.
+See L</Performance issues> above for the serious performance implications
+of using this variable (even once) in your code.
This variable is read-only and dynamically-scoped.
Mnemonic: C<'> often follows a quoted string.
=item ${^POSTMATCH}
-X<${^POSTMATCH}>
+X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
-performance penalty associated with that variable, and is only guaranteed
+performance penalty associated with that variable.
+
+See L</Performance issues> above.
+
+In Perl v5.18 and earlier, it is only guaranteed
to return a defined value when the pattern was compiled or executed with
-the C</p> modifier.
+the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
+C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
-This variable was added in Perl 5.10.
+This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
The text matched by the last bracket of the last successful search pattern.
This is useful if you don't know which one of a set of alternative patterns
-matched. For example:
+matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
=item $LAST_SUBMATCH_RESULT
=item $^N
-X<$^N>
+X<$^N> X<$LAST_SUBMATCH_RESULT>
The text matched by the used group most-recently closed (i.e. the group
with the rightmost closing parenthesis) of the last successful search
pattern.
This is primarily used inside C<(?{...})> blocks for examining text
-recently matched. For example, to effectively capture text to a variable
+recently matched. For example, to effectively capture text to a variable
(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
- (?:(...)(?{ $var = $^N }))
+ (?:(...)(?{ $var = $^N }))
By setting and then using C<$var> in this way relieves you from having to
worry about exactly which numbered set of parentheses they are.
-This variable was added in Perl 5.8.
+This variable was added in Perl v5.8.0.
Mnemonic: the (possibly) Nested parenthesis that most recently closed.
X<@+> X<@LAST_MATCH_END>
This array holds the offsets of the ends of the last successful
-submatches in the currently active dynamic scope. C<$+[0]> is
-the offset into the string of the end of the entire match. This
+submatches in the currently active dynamic scope. C<$+[0]> is
+the offset into the string of the end of the entire match. This
is the same value as what the C<pos> function returns when called
-on the variable that was matched against. The I<n>th element
+on the variable that was matched against. The I<n>th element
of this array holds the offset of the I<n>th submatch, so
C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
past where C<$2> ends, and so on. You can use C<$#+> to determine
-how many subgroups were in the last successful match. See the
+how many subgroups were in the last successful match. See the
examples given for the C<@-> variable.
-This variable was added in Perl 5.6.
+This variable was added in Perl v5.6.0.
=item %LAST_PAREN_MATCH
=item %+
-X<%+>
+X<%+> X<%LAST_PAREN_MATCH>
Similar to C<@+>, the C<%+> hash allows access to the named capture
buffers, should they exist, in the last successful match in the
For example, C<$+{foo}> is equivalent to C<$1> after the following match:
- 'foo' =~ /(?<foo>foo)/;
+ 'foo' =~ /(?<foo>foo)/;
The keys of the C<%+> hash list only the names of buffers that have
captured (and that are thus associated to defined values).
L<Tie::Hash::NamedCapture> module.
B<Note:> C<%-> and C<%+> are tied views into a common internal hash
-associated with the last successful regular expression. Therefore mixing
+associated with the last successful regular expression. Therefore mixing
iterative access to them via C<each> may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.
-This variable was added in Perl 5.10.
+This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
I<n>-th subpattern, or undef if the subpattern did not match.
Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
-$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
+$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
-C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
-matched subgroup in the last successful match. Contrast with
-C<$#+>, the number of subgroups in the regular expression. Compare
+C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
+last matched subgroup in the last successful match. Contrast with
+C<$#+>, the number of subgroups in the regular expression. Compare
with C<@+>.
This array holds the offsets of the beginnings of the last
=back
-This variable was added in Perl 5.6.
+This variable was added in Perl v5.6.0.
+
+=item %LAST_MATCH_START
=item %-
-X<%->
+X<%-> X<%LAST_MATCH_START>
Similar to C<%+>, this variable allows access to the named capture groups
-in the last successful match in the currently active dynamic scope. To
+in the last successful match in the currently active dynamic scope. To
each capture group name found in the regular expression, it associates a
reference to an array containing the list of values captured by all
buffers with that name (should there be several of them), in the order
my $ary = $-{$bufname};
foreach my $idx (0..$#$ary) {
print "\$-{$bufname}[$idx] : ",
- (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
+ (defined($ary->[$idx])
+ ? "'$ary->[$idx]'"
+ : "undef"),
"\n";
}
}
L<Tie::Hash::NamedCapture> module.
B<Note:> C<%-> and C<%+> are tied views into a common internal hash
-associated with the last successful regular expression. Therefore mixing
+associated with the last successful regular expression. Therefore mixing
iterative access to them via C<each> may have unpredictable results.
Likewise, if the last successful match changes, then the results may be
surprising.
-This variable was added in Perl 5.10
+This variable was added in Perl v5.10.0.
This variable is read-only and dynamically-scoped.
X<$^R> X<$LAST_REGEXP_CODE_RESULT>
The result of evaluation of the last successful C<(?{ code })>
-regular expression assertion (see L<perlre>). May be written to.
+regular expression assertion (see L<perlre>). May be written to.
This variable was added in Perl 5.005.
=item ${^RE_DEBUG_FLAGS}
+X<${^RE_DEBUG_FLAGS}>
-The current value of the regex debugging flags. Set to 0 for no debug output
-even when the C<re 'debug'> module is loaded. See L<re> for details.
+The current value of the regex debugging flags. Set to 0 for no debug output
+even when the C<re 'debug'> module is loaded. See L<re> for details.
-This variable was added in Perl 5.10.
+This variable was added in Perl v5.10.0.
=item ${^RE_TRIE_MAXBUF}
+X<${^RE_TRIE_MAXBUF}>
Controls how certain regex optimisations are applied and how much memory they
-utilize. This value by default is 65536 which corresponds to a 512kB temporary
-cache. Set this to a higher value to trade memory for speed when matching
-large alternations. Set it to a lower value if you want the optimisations to
+utilize. This value by default is 65536 which corresponds to a 512kB
+temporary cache. Set this to a higher value to trade
+memory for speed when matching large alternations. Set
+it to a lower value if you want the optimisations to
be as conservative of memory as possible but still occur, and set it to a
negative value to prevent the optimisation and conserve the most memory.
Under normal situations this variable should be of no interest to you.
-This variable was added in Perl 5.10.
+This variable was added in Perl v5.10.0.
=back
Variables that depend on the currently selected filehandle may be set
by calling an appropriate object method on the C<IO::Handle> object,
although this is less efficient than using the regular built-in
-variables. (Summary lines below for this contain the word HANDLE.)
+variables. (Summary lines below for this contain the word HANDLE.)
First you must say
use IO::Handle;
HANDLE->method(EXPR)
-Each method returns the old value of the C<IO::Handle> attribute. The
+Each method returns the old value of the C<IO::Handle> attribute. The
methods each take an optional EXPR, which, if supplied, specifies the
-new value for the C<IO::Handle> attribute in question. If not
+new value for the C<IO::Handle> attribute in question. If not
supplied, most methods do nothing to the current value--except for
C<autoflush()>, which will assume a 1 for you, just to be different.
Because loading in the C<IO::Handle> class is an expensive operation,
you should learn how to use the regular built-in variables.
-A few of these variables are considered "read-only". This means that
+A few of these variables are considered "read-only". This means that
if you try to assign to this variable, either directly or indirectly
through a reference, you'll raise a run-time exception.
You should be very careful when modifying the default values of most
-special variables described in this document. In most cases you want
+special variables described in this document. In most cases you want
to localize these variables before changing them, since if you don't,
the change may affect other modules which rely on the default values
-of the special variables that you have changed. This is one of the
+of the special variables that you have changed. This is one of the
correct ways to read the whole file at once:
open my $fh, "<", "foo" or die $!;
running inside the same Perl interpreter.
Usually when a variable is localized you want to make sure that this
-change affects the shortest scope possible. So unless you are already
-inside some short C<{}> block, you should create one yourself. For
+change affects the shortest scope possible. So unless you are already
+inside some short C<{}> block, you should create one yourself. For
example:
my $content = '';
open my $fh, "<", "foo" or die $!;
{
- local $/;
- $content = <$fh>;
+ local $/;
+ $content = <$fh>;
}
close $fh;
Here is an example of how your own code can go broken:
- for (1..5){
- nasty_break();
- print "$_ ";
+ for ( 1..3 ){
+ $\ = "\r\n";
+ nasty_break();
+ print "$_";
}
+
sub nasty_break {
- $_ = 5;
- # do something with $_
+ $\ = "\f";
+ # do something with $_
}
-You probably expect this code to print:
+You probably expect this code to print the equivalent of
- 1 2 3 4 5
+ "1\r\n2\r\n3\r\n"
but instead you get:
- 5 5 5 5 5
+ "1\f2\f3\f"
-Why? Because C<nasty_break()> modifies C<$_> without localizing it
-first. The fix is to add C<local()>:
+Why? Because C<nasty_break()> modifies C<$\> without localizing it
+first. The value you set in C<nasty_break()> is still there when you
+return. The fix is to add C<local()> so the value doesn't leak out of
+C<nasty_break()>:
- local $_ = 5;
+ local $\ = "\f";
It's easy to notice the problem in such a short example, but in more
complicated code you are looking for trouble if you don't localize
=over 8
-=item ARGV
-X<ARGV>
-
-The special filehandle that iterates over command-line filenames in
-C<@ARGV>. Usually written as the null filehandle in the angle operator
-C<< <> >>. Note that currently C<ARGV> only has its magical effect
-within the C<< <> >> operator; elsewhere it is just a plain filehandle
-corresponding to the last file opened by C<< <> >>. In particular,
-passing C<\*ARGV> as a parameter to a function that expects a filehandle
-may not cause your function to automatically read the contents of all the
-files in C<@ARGV>.
-
=item $ARGV
X<$ARGV>
-contains the name of the current file when reading from <>.
+Contains the name of the current file when reading from C<< <> >>.
=item @ARGV
X<@ARGV>
-The array @ARGV contains the command-line arguments intended for
-the script. C<$#ARGV> is generally the number of arguments minus
+The array C<@ARGV> contains the command-line arguments intended for
+the script. C<$#ARGV> is generally the number of arguments minus
one, because C<$ARGV[0]> is the first argument, I<not> the program's
-command name itself. See C<$0> for the command name.
+command name itself. See L</$0> for the command name.
+
+=item ARGV
+X<ARGV>
+
+The special filehandle that iterates over command-line filenames in
+C<@ARGV>. Usually written as the null filehandle in the angle operator
+C<< <> >>. Note that currently C<ARGV> only has its magical effect
+within the C<< <> >> operator; elsewhere it is just a plain filehandle
+corresponding to the last file opened by C<< <> >>. In particular,
+passing C<\*ARGV> as a parameter to a function that expects a filehandle
+may not cause your function to automatically read the contents of all the
+files in C<@ARGV>.
=item ARGVOUT
X<ARGVOUT>
The special filehandle that points to the currently open output file
-when doing edit-in-place processing with B<-i>. Useful when you have
-to do a lot of inserting and don't want to keep modifying C<$_>. See
+when doing edit-in-place processing with B<-i>. Useful when you have
+to do a lot of inserting and don't want to keep modifying C<$_>. See
L<perlrun> for the B<-i> switch.
-=item HANDLE->input_line_number(EXPR)
+=item IO::Handle->output_field_separator( EXPR )
-=item $INPUT_LINE_NUMBER
+=item $OUTPUT_FIELD_SEPARATOR
-=item $NR
+=item $OFS
-=item $.
-X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
+=item $,
+X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
+
+The output field separator for the print operator. If defined, this
+value is printed between each of print's arguments. Default is C<undef>.
+
+You cannot call C<output_field_separator()> on a handle, only as a
+static method. See L<IO::Handle|IO::Handle>.
+
+Mnemonic: what is printed when there is a "," in your print statement.
+
+=item HANDLE->input_line_number( EXPR )
+
+=item $INPUT_LINE_NUMBER
+
+=item $NR
+
+=item $.
+X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
Current line number for the last filehandle accessed.
for that filehandle.
You can adjust the counter by assigning to C<$.>, but this will not
-actually move the seek pointer. I<Localizing C<$.> will not localize
-the filehandle's line count>. Instead, it will localize perl's notion
+actually move the seek pointer. I<Localizing C<$.> will not localize
+the filehandle's line count>. Instead, it will localize perl's notion
of which filehandle C<$.> is currently aliased to.
C<$.> is reset when the filehandle is closed, but B<not> when an open
-filehandle is reopened without an intervening C<close()>. For more
-details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
+filehandle is reopened without an intervening C<close()>. For more
+details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
an explicit close, line numbers increase across C<ARGV> files (but see
examples in L<perlfunc/eof>).
Mnemonic: many programs use "." to mean the current line number.
-=item IO::Handle->input_record_separator(EXPR)
+=item IO::Handle->input_record_separator( EXPR )
=item $INPUT_RECORD_SEPARATOR
=item $/
X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
-The input record separator, newline by default. This
-influences Perl's idea of what a "line" is. Works like B<awk>'s RS
-variable, including treating empty lines as a terminator if set to
-the null string (an empty line cannot contain any spaces
-or tabs). You may set it to a multi-character string to match a
-multi-character terminator, or to C<undef> to read through the end
-of file. Setting it to C<"\n\n"> means something slightly
-different than setting to C<"">, if the file contains consecutive
-empty lines. Setting to C<""> will treat two or more consecutive
-empty lines as a single empty line. Setting to C<"\n\n"> will
-blindly assume that the next input character belongs to the next
-paragraph, even if it's a newline.
+The input record separator, newline by default. This influences Perl's
+idea of what a "line" is. Works like B<awk>'s RS variable, including
+treating empty lines as a terminator if set to the null string (an
+empty line cannot contain any spaces or tabs). You may set it to a
+multi-character string to match a multi-character terminator, or to
+C<undef> to read through the end of file. Setting it to C<"\n\n">
+means something slightly different than setting to C<"">, if the file
+contains consecutive empty lines. Setting to C<""> will treat two or
+more consecutive empty lines as a single empty line. Setting to
+C<"\n\n"> will blindly assume that the next input character belongs to
+the next paragraph, even if it's a newline.
local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
Setting C<$/> to a reference to an integer, scalar containing an
integer, or scalar that's convertible to an integer will attempt to
read records instead of lines, with the maximum record size being the
-referenced integer. So this:
+referenced integer number of characters. So this:
local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, "<", $myfile or die $!;
local $_ = <$fh>;
-will read a record of no more than 32768 bytes from FILE. If you're
+will read a record of no more than 32768 characters from $fh. If you're
not reading from a record-oriented file (or your OS doesn't have
record-oriented files), then you'll likely get a full chunk of data
with every read. If a record is larger than the record size you've
set, you'll get the record back in pieces. Trying to set the record
-size to zero or less will cause reading in the (rest of the) whole file.
+size to zero or less is deprecated and will cause $/ to have the value
+of "undef", which will cause reading in the (rest of the) whole file.
-On VMS, record reads are done with the equivalent of C<sysread>,
-so it's best not to mix record and non-record reads on the same
-file. (This is unlikely to be a problem, because any file you'd
-want to read in record mode is probably unusable in line mode.)
-Non-VMS systems do normal I/O, so it's safe to mix record and
-non-record reads of a file.
+As of 5.19.9 setting C<$/> to any other form of reference will throw a
+fatal exception. This is in preparation for supporting new ways to set
+C<$/> in the future.
-See also L<perlport/"Newlines">. Also see C<$.>.
+On VMS only, record reads bypass PerlIO layers and any associated
+buffering, so you must not mix record and non-record reads on the
+same filehandle. Record mode mixes with line mode only when the
+same buffering layer is in use for both modes.
-Mnemonic: / delimits line boundaries when quoting poetry.
+You cannot call C<input_record_separator()> on a handle, only as a
+static method. See L<IO::Handle|IO::Handle>.
-=item HANDLE->autoflush(EXPR)
+See also L<perlport/"Newlines">. Also see L</$.>.
-=item $OUTPUT_AUTOFLUSH
+Mnemonic: / delimits line boundaries when quoting poetry.
-=item $|
-X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
+=item IO::Handle->output_record_separator( EXPR )
-If set to nonzero, forces a flush right away and after every write
-or print on the currently selected output channel. Default is 0
-(regardless of whether the channel is really buffered by the
-system or not; C<$|> tells you only whether you've asked Perl
-explicitly to flush after each write). STDOUT will
-typically be line buffered if output is to the terminal and block
-buffered otherwise. Setting this variable is useful primarily when
-you are outputting to a pipe or socket, such as when you are running
-a Perl program under B<rsh> and want to see the output as it's
-happening. This has no effect on input buffering. See L<perlfunc/getc>
-for that. See L<perldoc/select> on how to select the output channel.
-See also L<IO::Handle>.
+=item $OUTPUT_RECORD_SEPARATOR
-Mnemonic: when you want your pipes to be piping hot.
+=item $ORS
-=item IO::Handle->output_field_separator EXPR
+=item $\
+X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
-=item $OUTPUT_FIELD_SEPARATOR
+The output record separator for the print operator. If defined, this
+value is printed after the last of print's arguments. Default is C<undef>.
-=item $OFS
+You cannot call C<output_record_separator()> on a handle, only as a
+static method. See L<IO::Handle|IO::Handle>.
-=item $,
-X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
+Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
+Also, it's just like C<$/>, but it's what you get "back" from Perl.
-The output field separator for the print operator. If defined, this
-value is printed between each of print's arguments. Default is C<undef>.
+=item HANDLE->autoflush( EXPR )
-Mnemonic: what is printed when there is a "," in your print statement.
+=item $OUTPUT_AUTOFLUSH
-=item IO::Handle->output_record_separator EXPR
+=item $|
+X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
-=item $OUTPUT_RECORD_SEPARATOR
+If set to nonzero, forces a flush right away and after every write or
+print on the currently selected output channel. Default is 0
+(regardless of whether the channel is really buffered by the system or
+not; C<$|> tells you only whether you've asked Perl explicitly to
+flush after each write). STDOUT will typically be line buffered if
+output is to the terminal and block buffered otherwise. Setting this
+variable is useful primarily when you are outputting to a pipe or
+socket, such as when you are running a Perl program under B<rsh> and
+want to see the output as it's happening. This has no effect on input
+buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
+how to select the output channel. See also L<IO::Handle>.
-=item $ORS
+Mnemonic: when you want your pipes to be piping hot.
-=item $\
-X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
+=item ${^LAST_FH}
+X<${^LAST_FH}>
-The output record separator for the print operator. If defined, this
-value is printed after the last of print's arguments. Default is C<undef>.
+This read-only variable contains a reference to the last-read filehandle.
+This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
+This is the same handle that C<$.> and C<tell> and C<eof> without arguments
+use. It is also the handle used when Perl appends ", <STDIN> line 1" to
+an error or warning message.
-Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
-Also, it's just like C<$/>, but it's what you get "back" from Perl.
+This variable was added in Perl v5.18.0.
=back
-
=head3 Variables related to formats
The special variables for formats are a subset of those for
-filehandles so they have
-
-See L<perlform> for more information about Perl's formats.
+filehandles. See L<perlform> for more information about Perl's
+formats.
=over 8
+=item $ACCUMULATOR
+
+=item $^A
+X<$^A> X<$ACCUMULATOR>
+
+The current value of the C<write()> accumulator for C<format()> lines.
+A format contains C<formline()> calls that put their result into
+C<$^A>. After calling its format, C<write()> prints out the contents
+of C<$^A> and empties. So you never really see the contents of C<$^A>
+unless you call C<formline()> yourself and then look at it. See
+L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
+
+=item IO::Handle->format_formfeed(EXPR)
+
+=item $FORMAT_FORMFEED
+
+=item $^L
+X<$^L> X<$FORMAT_FORMFEED>
+
+What formats output as a form feed. The default is C<\f>.
+
+You cannot call C<format_formfeed()> on a handle, only as a static
+method. See L<IO::Handle|IO::Handle>.
+
=item HANDLE->format_page_number(EXPR)
=item $FORMAT_PAGE_NUMBER
Mnemonic: C<%> is page number in B<nroff>.
-=item HANDLE->format_lines_per_page(EXPR)
-
-=item $FORMAT_LINES_PER_PAGE
-
-=item $=
-X<$=> X<$FORMAT_LINES_PER_PAGE>
-
-The current page length (printable lines) of the currently selected
-output channel. The default is 60.
-
-Mnemonic: = has horizontal lines.
-
=item HANDLE->format_lines_left(EXPR)
=item $FORMAT_LINES_LEFT
Mnemonic: lines_on_page - lines_printed.
-=item HANDLE->format_name(EXPR)
-
-=item $FORMAT_NAME
+=item IO::Handle->format_line_break_characters EXPR
-=item $~
-X<$~> X<$FORMAT_NAME>
+=item $FORMAT_LINE_BREAK_CHARACTERS
-The name of the current report format for the currently selected
-output channel. The default format name is the same as the filehandle
-name. For example, the default format name for the C<STDOUT>
-filehandle is just C<STDOUT>.
+=item $:
+X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
-Mnemonic: brother to C<$^>.
+The current set of characters after which a string may be broken to
+fill continuation fields (starting with C<^>) in a format. The default is
+S<" \n-">, to break on a space, newline, or a hyphen.
-=item HANDLE->format_top_name(EXPR)
+You cannot call C<format_line_break_characters()> on a handle, only as
+a static method. See L<IO::Handle|IO::Handle>.
-=item $FORMAT_TOP_NAME
+Mnemonic: a "colon" in poetry is a part of a line.
-=item $^
-X<$^> X<$FORMAT_TOP_NAME>
+=item HANDLE->format_lines_per_page(EXPR)
-The name of the current top-of-page format for the currently selected
-output channel. The default is the name of the filehandle with C<_TOP>
-appended. For example, the default format top name for the C<STDOUT>
-filehanlde is C<STDOUT_TOP>.
+=item $FORMAT_LINES_PER_PAGE
-Mnemonic: points to top of page.
+=item $=
+X<$=> X<$FORMAT_LINES_PER_PAGE>
-=item IO::Handle->format_line_break_characters EXPR
+The current page length (printable lines) of the currently selected
+output channel. The default is 60.
-=item $FORMAT_LINE_BREAK_CHARACTERS
+Mnemonic: = has horizontal lines.
-=item $:
-X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
+=item HANDLE->format_top_name(EXPR)
-The current set of characters after which a string may be broken to
-fill continuation fields (starting with C<^>) in a format. The default is
-S<" \n-">, to break on a space, newline, or a hyphen.
+=item $FORMAT_TOP_NAME
-Mnemonic: a "colon" in poetry is a part of a line.
+=item $^
+X<$^> X<$FORMAT_TOP_NAME>
-=item IO::Handle->format_formfeed EXPR
+The name of the current top-of-page format for the currently selected
+output channel. The default is the name of the filehandle with C<_TOP>
+appended. For example, the default format top name for the C<STDOUT>
+filehandle is C<STDOUT_TOP>.
-=item $FORMAT_FORMFEED
+Mnemonic: points to top of page.
-=item $^L
-X<$^L> X<$FORMAT_FORMFEED>
+=item HANDLE->format_name(EXPR)
-What formats output as a form feed. The default is C<\f>.
+=item $FORMAT_NAME
-=item $ACCUMULATOR
+=item $~
+X<$~> X<$FORMAT_NAME>
-=item $^A
-X<$^A> X<$ACCUMULATOR>
+The name of the current report format for the currently selected
+output channel. The default format name is the same as the filehandle
+name. For example, the default format name for the C<STDOUT>
+filehandle is just C<STDOUT>.
-The current value of the C<write()> accumulator for C<format()> lines.
-A format contains C<formline()> calls that put their result into
-C<$^A>. After calling its format, C<write()> prints out the contents
-of C<$^A> and empties. So you never really see the contents of C<$^A>
-unless you call C<formline()> yourself and then look at it. See
-L<perlform> and L<perlfunc/formline()>.
+Mnemonic: brother to C<$^>.
=back
-=head2 Error Indicators
+=head2 Error Variables
X<error> X<exception>
The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
about different types of error conditions that may appear during
-execution of a Perl program. The variables are shown ordered by
+execution of a Perl program. The variables are shown ordered by
the "distance" between the subsystem which reported the error and
-the Perl process. They correspond to errors detected by the Perl
+the Perl process. They correspond to errors detected by the Perl
interpreter, C library, operating system, or an external program,
respectively.
To illustrate the differences between these variables, consider the
-following Perl expression, which uses a single-quoted string:
+following Perl expression, which uses a single-quoted string. After
+execution of this statement, perl may have set all four special error
+variables:
eval q{
open my $pipe, "/cdrom/install |" or die $!;
close $pipe or die "bad pipe: $?, $!";
};
-After execution of this statement all 4 variables may have been set.
+When perl executes the C<eval()> expression, it translates the
+C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
+and thence to the operating system kernel. perl sets C<$!> to
+the C library's C<errno> if one of these calls fails.
-C<$@> is set if the string to be C<eval>-ed did not compile (this
-may happen if C<open> or C<close> were imported with bad prototypes),
-or if Perl code executed during evaluation die()d . In these cases
-the value of $@ is the compile error, or the argument to C<die>
-(which will interpolate C<$!> and C<$?>). (See also L<Fatal>,
-though.)
+C<$@> is set if the string to be C<eval>-ed did not compile (this may
+happen if C<open> or C<close> were imported with bad prototypes), or
+if Perl code executed during evaluation C<die()>d. In these cases the
+value of C<$@> is the compile error, or the argument to C<die> (which
+will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
-When the C<eval()> expression above is executed, C<open()>, C<< <PIPE> >>,
-and C<close> are translated to calls in the C run-time library and
-thence to the operating system kernel. C<$!> is set to the C library's
-C<errno> if one of these calls fails.
-
-Under a few operating systems, C<$^E> may contain a more verbose
-error indicator, such as in this case, "CDROM tray not closed."
-Systems that do not support extended error messages leave C<$^E>
-the same as C<$!>.
+Under a few operating systems, C<$^E> may contain a more verbose error
+indicator, such as in this case, "CDROM tray not closed." Systems that
+do not support extended error messages leave C<$^E> the same as C<$!>.
Finally, C<$?> may be set to non-0 value if the external program
-F</cdrom/install> fails. The upper eight bits reflect specific
-error conditions encountered by the program (the program's C<exit()>
-value). The lower eight bits reflect mode of failure, like signal
-death and core dump information. See C<wait(2)> for details. In
-contrast to C<$!> and C<$^E>, which are set only if error condition
-is detected, the variable C<$?> is set on each C<wait> or pipe
-C<close>, overwriting the old value. This is more like C<$@>, which
-on every C<eval()> is always set on failure and cleared on success.
+F</cdrom/install> fails. The upper eight bits reflect specific error
+conditions encountered by the program (the program's C<exit()> value).
+The lower eight bits reflect mode of failure, like signal death and
+core dump information. See L<wait(2)> for details. In contrast to
+C<$!> and C<$^E>, which are set only if error condition is detected,
+the variable C<$?> is set on each C<wait> or pipe C<close>,
+overwriting the old value. This is more like C<$@>, which on every
+C<eval()> is always set on failure and cleared on success.
For more details, see the individual descriptions at C<$@>, C<$!>,
C<$^E>, and C<$?>.
+=over 8
+
+=item ${^CHILD_ERROR_NATIVE}
+X<$^CHILD_ERROR_NATIVE>
+
+The native status returned by the last pipe close, backtick (C<``>)
+command, successful call to C<wait()> or C<waitpid()>, or from the
+C<system()> operator. On POSIX-like systems this value can be decoded
+with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
+WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
+
+Under VMS this reflects the actual VMS exit status; i.e. it is the
+same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
+
+This variable was added in Perl v5.10.0.
+
+=item $EXTENDED_OS_ERROR
+
+=item $^E
+X<$^E> X<$EXTENDED_OS_ERROR>
+
+Error information specific to the current operating system. At the
+moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
+for MacPerl). On all other platforms, C<$^E> is always just the same
+as C<$!>.
+
+Under VMS, C<$^E> provides the VMS status value from the last system
+error. This is more specific information about the last system error
+than that provided by C<$!>. This is particularly important when C<$!>
+is set to B<EVMSERR>.
+
+Under OS/2, C<$^E> is set to the error code of the last call to OS/2
+API either via CRT, or directly from perl.
+
+Under Win32, C<$^E> always returns the last error information reported
+by the Win32 call C<GetLastError()> which describes the last error
+from within the Win32 API. Most Win32-specific code will report errors
+via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
+portable Perl code will report errors via C<$!>.
+
+Caveats mentioned in the description of C<$!> generally apply to
+C<$^E>, also.
+
+This variable was added in Perl 5.003.
+
+Mnemonic: Extra error explanation.
+
+=item $EXCEPTIONS_BEING_CAUGHT
+
+=item $^S
+X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
+
+Current state of the interpreter.
+
+ $^S State
+ --------- -------------------------------------
+ undef Parsing module, eval, or main program
+ true (1) Executing an eval
+ false (0) Otherwise
+
+The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
+handlers.
+
+The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
+the C<undef> value does not indicate whether exceptions are being caught,
+since compilation of the main program does not catch exceptions.
+
+This variable was added in Perl 5.004.
+
+=item $WARNING
+
+=item $^W
+X<$^W> X<$WARNING>
+
+The current value of the warning switch, initially true if B<-w> was
+used, false otherwise, but directly modifiable.
+
+See also L<warnings>.
+
+Mnemonic: related to the B<-w> switch.
+
+=item ${^WARNING_BITS}
+X<${^WARNING_BITS}>
+
+The current set of warning checks enabled by the C<use warnings> pragma.
+It has the same scoping as the C<$^H> and C<%^H> variables. The exact
+values are considered internal to the L<warnings> pragma and may change
+between versions of Perl.
+
+This variable was added in Perl v5.6.0.
+
+=item $OS_ERROR
+
+=item $ERRNO
+
+=item $!
+X<$!> X<$ERRNO> X<$OS_ERROR>
+
+When referenced, C<$!> retrieves the current value
+of the C C<errno> integer variable.
+If C<$!> is assigned a numerical value, that value is stored in C<errno>.
+When referenced as a string, C<$!> yields the system error string
+corresponding to C<errno>.
+
+Many system or library calls set C<errno> if they fail,
+to indicate the cause of failure. They usually do B<not>
+set C<errno> to zero if they succeed. This means C<errno>,
+hence C<$!>, is meaningful only I<immediately> after a B<failure>:
+
+ if (open my $fh, "<", $filename) {
+ # Here $! is meaningless.
+ ...
+ }
+ else {
+ # ONLY here is $! meaningful.
+ ...
+ # Already here $! might be meaningless.
+ }
+ # Since here we might have either success or failure,
+ # $! is meaningless.
+
+Here, I<meaningless> means that C<$!> may be unrelated to the outcome
+of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
+It can be used immediately before invoking the C<die()> operator,
+to set the exit value, or to inspect the system error string
+corresponding to error I<n>, or to restore C<$!> to a meaningful state.
+
+Mnemonic: What just went bang?
+
+=item %OS_ERROR
+
+=item %ERRNO
+
+=item %!
+X<%!> X<%OS_ERROR> X<%ERRNO>
+
+Each element of C<%!> has a true value only if C<$!> is set to that
+value. For example, C<$!{ENOENT}> is true if and only if the current
+value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
+such file or directory" (or its moral equivalent: not all operating
+systems give that exact error, and certainly not all languages). To
+check if a particular key is meaningful on your system, use C<exists
+$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
+for more information, and also see L</$!>.
+
+This variable was added in Perl 5.005.
+
=item $CHILD_ERROR
=item $?
The status returned by the last pipe close, backtick (C<``>) command,
successful call to C<wait()> or C<waitpid()>, or from the C<system()>
-operator. This is just the 16-bit status word returned by the
+operator. This is just the 16-bit status word returned by the
traditional Unix C<wait()> system call (or else is made up to look
-like it). Thus, the exit value of the subprocess is really (C<<< $? >>
+like it). Thus, the exit value of the subprocess is really (C<<< $? >>
8 >>>), and C<$? & 127> gives which signal, if any, the process died
from, and C<$? & 128> reports whether there was a core dump.
value of C<$?> will usually be wrong outside that handler.
Inside an C<END> subroutine C<$?> contains the value that is going to be
-given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
-change the exit status of your program. For example:
+given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
+change the exit status of your program. For example:
END {
$? = 1 if $? == 255; # die would make it 255
actual VMS exit status, instead of the default emulation of POSIX
status; see L<perlvms/$?> for details.
-Also see L<Error Indicators>.
-
Mnemonic: similar to B<sh> and B<ksh>.
-=item ${^CHILD_ERROR_NATIVE}
-X<$^CHILD_ERROR_NATIVE>
+=item $EVAL_ERROR
-The native status returned by the last pipe close, backtick (C<``>)
-command, successful call to C<wait()> or C<waitpid()>, or from the
-C<system()> operator. On POSIX-like systems this value can be decoded
-with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
-WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
+=item $@
+X<$@> X<$EVAL_ERROR>
-Under VMS this reflects the actual VMS exit status; i.e. it is the
-same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
+The Perl syntax error message from the
+last C<eval()> operator. If C<$@> is
+the null string, the last C<eval()> parsed and executed correctly
+(although the operations you invoked may have failed in the normal
+fashion).
-This variable was added in Perl 5.8.9.
+Warning messages are not collected in this variable. You can, however,
+set up a routine to process warnings by setting C<$SIG{__WARN__}> as
+described in L</%SIG>.
-=item $OS_ERROR
+Mnemonic: Where was the syntax error "at"?
-=item $ERRNO
+=back
-=item $!
-X<$!> X<$ERRNO> X<$OS_ERROR>
+=head2 Variables related to the interpreter state
-If used numerically, yields the current value of the C C<errno>
-variable, or in other words, if a system or library call fails, it
-sets this variable. This means that the value of C<$!> is meaningful
-only I<immediately> after a B<failure>:
+These variables provide information about the current interpreter state.
- if (open my $fh, "<", $filename) {
- # Here $! is meaningless.
- ...
- } else {
- # ONLY here is $! meaningful.
- ...
- # Already here $! might be meaningless.
+=over 8
+
+=item $COMPILING
+
+=item $^C
+X<$^C> X<$COMPILING>
+
+The current value of the flag associated with the B<-c> switch.
+Mainly of use with B<-MO=...> to allow code to alter its behavior
+when being compiled, such as for example to C<AUTOLOAD> at compile
+time rather than normal, deferred loading. Setting
+C<$^C = 1> is similar to calling C<B::minus_c>.
+
+This variable was added in Perl v5.6.0.
+
+=item $DEBUGGING
+
+=item $^D
+X<$^D> X<$DEBUGGING>
+
+The current value of the debugging flags. May be read or set. Like its
+command-line equivalent, you can use numeric or symbolic values, eg
+C<$^D = 10> or C<$^D = "st">.
+
+Mnemonic: value of B<-D> switch.
+
+=item ${^ENCODING}
+X<${^ENCODING}>
+
+DEPRECATED!!!
+
+The I<object reference> to the C<Encode> object that is used to convert
+the source code to Unicode. Thanks to this variable your Perl script
+does not have to be written in UTF-8. Default is C<undef>.
+
+Setting this variable to any other value than C<undef> is deprecated due
+to fundamental defects in its design and implementation. It is planned
+to remove it from a future Perl version. Its purpose was to allow your
+non-ASCII Perl scripts to not have to be written in UTF-8; this was
+useful before editors that worked on UTF-8 encoded text were common, but
+that was long ago. It causes problems, such as affecting the operation
+of other modules that aren't expecting it, causing general mayhem. Its
+use can lead to segfaults.
+
+If you need something like this functionality, you should use the
+L<encoding> pragma, which is also deprecated, but has fewer nasty side
+effects.
+
+If you are coming here because code of yours is being adversely affected
+by someone's use of this variable, you can usually work around it by
+doing this:
+
+ local ${^ENCODING};
+
+near the beginning of the functions that are getting broken. This
+undefines the variable during the scope of execution of the including
+function.
+
+This variable was added in Perl 5.8.2.
+
+=item ${^GLOBAL_PHASE}
+X<${^GLOBAL_PHASE}>
+
+The current phase of the perl interpreter.
+
+Possible values are:
+
+=over 8
+
+=item CONSTRUCT
+
+The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
+value is mostly there for completeness and for use via the
+underlying C variable C<PL_phase>. It's not really possible for Perl
+code to be executed unless construction of the interpreter is
+finished.
+
+=item START
+
+This is the global compile-time. That includes, basically, every
+C<BEGIN> block executed directly or indirectly from during the
+compile-time of the top-level program.
+
+This phase is not called "BEGIN" to avoid confusion with
+C<BEGIN>-blocks, as those are executed during compile-time of any
+compilation unit, not just the top-level program. A new, localised
+compile-time entered at run-time, for example by constructs as
+C<eval "use SomeModule"> are not global interpreter phases, and
+therefore aren't reflected by C<${^GLOBAL_PHASE}>.
+
+=item CHECK
+
+Execution of any C<CHECK> blocks.
+
+=item INIT
+
+Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
+
+=item RUN
+
+The main run-time, i.e. the execution of C<PL_main_root>.
+
+=item END
+
+Execution of any C<END> blocks.
+
+=item DESTRUCT
+
+Global destruction.
+
+=back
+
+Also note that there's no value for UNITCHECK-blocks. That's because
+those are run for each compilation unit individually, and therefore is
+not a global interpreter phase.
+
+Not every program has to go through each of the possible phases, but
+transition from one phase to another can only happen in the order
+described in the above list.
+
+An example of all of the phases Perl code can see:
+
+ BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
+
+ INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
+
+ CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
+
+ {
+ package Print::Phase;
+
+ sub new {
+ my ($class, $time) = @_;
+ return bless \$time, $class;
+ }
+
+ sub DESTROY {
+ my $self = shift;
+ print "$$self: ${^GLOBAL_PHASE}\n";
+ }
}
- # Since here we might have either success or failure,
- # here $! is meaningless.
-In the above I<meaningless> stands for anything: zero, non-zero,
-C<undef>. A successful system or library call does B<not> set
-the variable to zero.
+ print "run-time: ${^GLOBAL_PHASE}\n";
-If used as a string, yields the corresponding system error string.
-You can assign a number to C<$!> to set I<errno> if, for instance,
-you want C<"$!"> to return the string for error I<n>, or you want
-to set the exit value for the C<die()> operator.
+ my $runtime = Print::Phase->new(
+ "lexical variables are garbage collected before END"
+ );
-Also see L<Error Indicators>.
+ END { print "end-time: ${^GLOBAL_PHASE}\n" }
-Mnemonic: What just went bang?
+ our $destruct = Print::Phase->new(
+ "package variables are garbage collected after END"
+ );
-=item %OS_ERROR
+This will print out
-=item %ERRNO
+ compile-time: START
+ check-time: CHECK
+ init-time: INIT
+ run-time: RUN
+ lexical variables are garbage collected before END: RUN
+ end-time: END
+ package variables are garbage collected after END: DESTRUCT
-=item %!
-X<%!>
+This variable was added in Perl 5.14.0.
-Each element of C<%!> has a true value only if C<$!> is set to that
-value. For example, C<$!{ENOENT}> is true if and only if the current
-value of C<$!> is C<ENOENT>; that is, if the most recent error was
-"No such file or directory" (or its moral equivalent: not all operating
-systems give that exact error, and certainly not all languages).
-To check if a particular key is meaningful on your system, use
-C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
-See L<Errno> for more information, and also see above for the
-validity of C<$!>.
+=item $^H
+X<$^H>
-This variable was added in Perl 5.005.
+WARNING: This variable is strictly for
+internal use only. Its availability,
+behavior, and contents are subject to change without notice.
-=item $EXTENDED_OS_ERROR
+This variable contains compile-time hints for the Perl interpreter. At the
+end of compilation of a BLOCK the value of this variable is restored to the
+value when the interpreter started to compile the BLOCK.
-=item $^E
-X<$^E> X<$EXTENDED_OS_ERROR>
+When perl begins to parse any block construct that provides a lexical scope
+(e.g., eval body, required file, subroutine body, loop body, or conditional
+block), the existing value of C<$^H> is saved, but its value is left unchanged.
+When the compilation of the block is completed, it regains the saved value.
+Between the points where its value is saved and restored, code that
+executes within BEGIN blocks is free to change the value of C<$^H>.
-Error information specific to the current operating system. At
-the moment, this differs from C<$!> under only VMS, OS/2, and Win32
-(and for MacPerl). On all other platforms, C<$^E> is always just
-the same as C<$!>.
+This behavior provides the semantic of lexical scoping, and is used in,
+for instance, the C<use strict> pragma.
-Under VMS, C<$^E> provides the VMS status value from the last
-system error. This is more specific information about the last
-system error than that provided by C<$!>. This is particularly
-important when C<$!> is set to B<EVMSERR>.
+The contents should be an integer; different bits of it are used for
+different pragmatic flags. Here's an example:
-Under OS/2, C<$^E> is set to the error code of the last call to
-OS/2 API either via CRT, or directly from perl.
+ sub add_100 { $^H |= 0x100 }
-Under Win32, C<$^E> always returns the last error information
-reported by the Win32 call C<GetLastError()> which describes
-the last error from within the Win32 API. Most Win32-specific
-code will report errors via C<$^E>. ANSI C and Unix-like calls
-set C<errno> and so most portable Perl code will report errors
-via C<$!>.
+ sub foo {
+ BEGIN { add_100() }
+ bar->baz($boon);
+ }
-Caveats mentioned in the description of C<$!> generally apply to
-C<$^E>, also.
+Consider what happens during execution of the BEGIN block. At this point
+the BEGIN block has already been compiled, but the body of C<foo()> is still
+being compiled. The new value of C<$^H>
+will therefore be visible only while
+the body of C<foo()> is being compiled.
+
+Substitution of C<BEGIN { add_100() }> block with:
+
+ BEGIN { require strict; strict->import('vars') }
+
+demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
+version of the same lexical pragma:
+
+ BEGIN {
+ require strict; strict->import('vars') if $condition
+ }
This variable was added in Perl 5.003.
-Mnemonic: Extra error explanation.
-
-=item $EVAL_ERROR
+=item %^H
+X<%^H>
-=item $@
-X<$@> X<$EVAL_ERROR>
+The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
+it useful for implementation of lexically scoped pragmas. See
+L<perlpragma>. All the entries are stringified when accessed at
+runtime, so only simple values can be accommodated. This means no
+pointers to objects, for example.
-The Perl syntax error message from the last eval() operator. If $@ is
-the null string, the last eval() parsed and executed correctly
-(although the operations you invoked may have failed in the normal
-fashion).
+When putting items into C<%^H>, in order to avoid conflicting with other
+users of the hash there is a convention regarding which keys to use.
+A module should use only keys that begin with the module's name (the
+name of its main package) and a "/" character. For example, a module
+C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
-Warning messages are not collected in this variable. You can, however,
-set up a routine to process warnings by setting C<$SIG{__WARN__}> as
-described below.
+This variable was added in Perl v5.6.0.
-Also see L<Error Indicators>.
+=item ${^OPEN}
+X<${^OPEN}>
-Mnemonic: Where was the syntax error "at"?
-
-=item $EXCEPTIONS_BEING_CAUGHT
+An internal variable used by PerlIO. A string in two parts, separated
+by a C<\0> byte, the first part describes the input layers, the second
+part describes the output layers.
-=item $^S
-X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
+This variable was added in Perl v5.8.0.
-Current state of the interpreter.
+=item $PERLDB
- $^S State
- --------- -------------------
- undef Parsing module/eval
- true (1) Executing an eval
- false (0) Otherwise
+=item $^P
+X<$^P> X<$PERLDB>
-The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}> handlers.
+The internal variable for debugging support. The meanings of the
+various bits are subject to change, but currently indicate:
-This variable was added in Perl 5.004.
+=over 6
-=item $WARNING
+=item 0x01
-=item $^W
-X<$^W> X<$WARNING>
+Debug subroutine enter/exit.
-The current value of the warning switch, initially true if B<-w> was
-used, false otherwise, but directly modifiable.
+=item 0x02
-See also L<warnings>.
+Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
+each statement executed. Also causes saving source code lines (like
+0x400).
-Mnemonic: related to the B<-w> switch.
+=item 0x04
-=item ${^WARNING_BITS}
+Switch off optimizations.
-The current set of warning checks enabled by the C<use warnings> pragma.
-See the documentation of C<warnings> for more details.
+=item 0x08
+
+Preserve more data for future interactive inspections.
-This variable was added in Perl 5.10.
+=item 0x10
+
+Keep info about source lines on which a subroutine is defined.
+
+=item 0x20
+
+Start with single-step on.
+
+=item 0x40
+
+Use subroutine address instead of name when reporting.
+
+=item 0x80
+
+Report C<goto &subroutine> as well.
+
+=item 0x100
+
+Provide informative "file" names for evals based on the place they were compiled.
+
+=item 0x200
+
+Provide informative names to anonymous subroutines based on the place they
+were compiled.
+
+=item 0x400
+
+Save source code lines into C<@{"_<$filename"}>.
+
+=item 0x800
+
+When saving source, include evals that generate no subroutines.
+
+=item 0x1000
+
+When saving source, include source that did not compile.
=back
-=head2 Deprecated and removed variables
+Some bits may be relevant at compile-time only, some at
+run-time only. This is a new mechanism and the details may change.
+See also L<perldebguts>.
-Deprecating a variable announces the perl maintainers intent to
-eventually remove the varaible from the langauge. It may still be
-available despite its status. Using a deprecated variable triggers
-a warning.
+=item ${^TAINT}
+X<${^TAINT}>
-Once the variable is removed, its use triggers an error telling you
-the variable is unsupported.
+Reflects if taint mode is on or off. 1 for on (the program was run with
+B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
+B<-t> or B<-TU>).
-See L<perldiag> for details about the error messages.
+This variable is read-only.
-=over 8
+This variable was added in Perl v5.8.0.
-=item $*
-X<$*>
+=item ${^UNICODE}
+X<${^UNICODE}>
-C<$*> used to be a variable that enabled multiline matching.
-After a deprecation cycle, its magic was removed in Perl 5.10.
-Using it now triggers a warning: C<$* is no longer supported>.
-Use the C</s> and C</m> regexp modifiers instead.
+Reflects certain Unicode settings of Perl. See L<perlrun>
+documentation for the C<-C> switch for more information about
+the possible values.
-Deprecated in Perl 5.
+This variable is set during Perl startup and is thereafter read-only.
-Removed in Perl 5.10.
+This variable was added in Perl v5.8.2.
-=item $]
-X<$]>
+=item ${^UTF8CACHE}
+X<${^UTF8CACHE}>
-The version + patchlevel / 1000 of the Perl interpreter. This variable
-can be used to determine whether the Perl interpreter executing a
-script is in the right range of versions:
+This variable controls the state of the internal UTF-8 offset caching code.
+1 for on (the default), 0 for off, -1 to debug the caching code by checking
+all its results against linear scans, and panicking on any discrepancy.
- warn "No checksumming!\n" if $] < 3.019;
+This variable was added in Perl v5.8.9. It is subject to change or
+removal without notice, but is currently used to avoid recalculating the
+boundaries of multi-byte UTF-8-encoded characters.
-See also the documentation of C<use VERSION> and C<require VERSION>
-for a convenient way to fail if the running Perl interpreter is too old.
+=item ${^UTF8LOCALE}
+X<${^UTF8LOCALE}>
-The floating point representation can sometimes lead to inaccurate
-numeric comparisons. See C<$^V> for a more modern representation of
-the Perl version that allows accurate string comparisons.
+This variable indicates whether a UTF-8 locale was detected by perl at
+startup. This information is used by perl when it's in
+adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
+switch); see L<perlrun> for more info on this.
-Mnemonic: Is this version of perl in the right bracket?
+This variable was added in Perl v5.8.8.
+
+=back
-Deprecated in Perl 5.6.
+=head2 Deprecated and removed variables
+
+Deprecating a variable announces the intent of the perl maintainers to
+eventually remove the variable from the language. It may still be
+available despite its status. Using a deprecated variable triggers
+a warning.
+
+Once a variable is removed, its use triggers an error telling you
+the variable is unsupported.
+
+See L<perldiag> for details about error messages.
+
+=over 8
=item $#
X<$#>
-C<$#> used to be a variable that could be used to format printed numbers.
-After a deprecation cycle, its magic was removed in Perl 5.10 and using it
-now triggers a warning: C<$# is no longer supported>.
+C<$#> was a variable that could be used to format printed numbers.
+After a deprecation cycle, its magic was removed in Perl v5.10.0 and
+using it now triggers a warning: C<$# is no longer supported>.
This is not the sigil you use in front of an array name to get the
-last index, like C<$#array>. That's still how you get the last index
-of an array in Perl. The two have nothing to do with each other.
+last index, like C<$#array>. That's still how you get the last index
+of an array in Perl. The two have nothing to do with each other.
Deprecated in Perl 5.
-Removed in Perl 5.10.
+Removed in Perl v5.10.0.
+
+=item $*
+X<$*>
+
+C<$*> was a variable that you could use to enable multiline matching.
+After a deprecation cycle, its magic was removed in Perl v5.10.0.
+Using it now triggers a warning: C<$* is no longer supported>.
+You should use the C</s> and C</m> regexp modifiers instead.
+
+Deprecated in Perl 5.
+
+Removed in Perl v5.10.0.
=item $[
X<$[>
-The index of the first element in an array, and of the first character
-in a substring. You use to be able to assign to this variable, but you
-can't do that anymore. It's always 0, like God intended.
+This variable stores the index of the first element in an array, and
+of the first character in a substring. The default is 0, but you could
+theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
+when subscripting and when evaluating the index() and substr() functions.
+
+As of release 5 of Perl, assignment to C<$[> is treated as a compiler
+directive, and cannot influence the behavior of any other file.
+(That's why you can only assign compile-time constants to it.)
+Its use is highly discouraged.
+
+Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
+scopes in the same file, unlike other compile-time directives (such as
+L<strict>). Using local() on it would bind its value strictly to a lexical
+block. Now it is always lexically scoped.
+
+As of Perl v5.16.0, it is implemented by the L<arybase> module. See
+L<arybase> for more details on its behaviour.
+
+Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
+effect, and always contains 0. Assigning 0 to it is permitted, but any
+other value will produce an error.
Mnemonic: [ begins subscripts.
-Deprecated in Perl 5.12.
+Deprecated in Perl v5.12.0.
-=back
+=item $]
+X<$]>
-=head1 BUGS
+See L</$^V> for a more modern representation of the Perl version that allows
+accurate string comparisons.
-Due to an unfortunate accident of Perl's implementation, C<use
-English> imposes a considerable performance penalty on all regular
-expression matches in a program, regardless of whether they occur
-in the scope of C<use English>. For that reason, saying C<use
-English> in libraries is strongly discouraged. See the
-Devel::SawAmpersand module documentation from CPAN
-( http://www.cpan.org/modules/by-module/Devel/ )
-for more information. Writing C<use English '-no_match_vars';>
-avoids the performance penalty.
+The version + patchlevel / 1000 of the Perl interpreter. This variable
+can be used to determine whether the Perl interpreter executing a
+script is in the right range of versions:
-Having to even think about the C<$^S> variable in your exception
-handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
-invites grievous and difficult to track down errors. Avoid it
-and use an C<END{}> or CORE::GLOBAL::die override instead.
+ warn "No PerlIO!\n" if $] lt '5.008';
+
+The floating point representation can sometimes lead to inaccurate
+numeric comparisons, so string comparisons are recommended.
+
+See also the documentation of C<use VERSION> and C<require VERSION>
+for a convenient way to fail if the running Perl interpreter is too old.
+
+Mnemonic: Is this version of perl in the right bracket?
+
+=back
+
+=cut
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