3 perlvar - Perl predefined variables
7 =head2 The Syntax of Variable Names
9 Variable names in Perl can have several formats. Usually, they
10 must begin with a letter or underscore, in which case they can be
11 arbitrarily long (up to an internal limit of 251 characters) and
12 may contain letters, digits, underscores, or the special sequence
13 C<::> or C<'>. In this case, the part before the last C<::> or
14 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
15 A Unicode letter that is not ASCII is not considered to be a letter
16 unless S<C<"use utf8">> is in effect, and somewhat more complicated
17 rules apply; see L<perldata/Identifier parsing> for details.
19 Perl variable names may also be a sequence of digits, a single
20 punctuation character, or the two-character sequence: C<^> (caret or
21 CIRCUMFLEX ACCENT) followed by any one of the characters C<[][A-Z^_?\]>.
22 These names are all reserved for
23 special uses by Perl; for example, the all-digits names are used
24 to hold data captured by backreferences after a regular expression
27 Since Perl v5.6.0, Perl variable names may also be alphanumeric strings
28 preceded by a caret. These must all be written in the form C<${^Foo}>;
29 the braces are not optional. C<${^Foo}> denotes the scalar variable
30 whose name is considered to be a control-C<F> followed by two C<o>'s.
32 reserved for future special uses by Perl, except for the ones that
33 begin with C<^_> (caret-underscore). No
34 name that begins with C<^_> will acquire a special
35 meaning in any future version of Perl; such names may therefore be
36 used safely in programs. C<$^_> itself, however, I<is> reserved.
38 Perl identifiers that begin with digits or
39 punctuation characters are exempt from the effects of the C<package>
40 declaration and are always forced to be in package C<main>; they are
41 also exempt from C<strict 'vars'> errors. A few other names are also
50 In particular, the special C<${^_XYZ}> variables are always taken
51 to be in package C<main>, regardless of any C<package> declarations
54 =head1 SPECIAL VARIABLES
56 The following names have special meaning to Perl. Most punctuation
57 names have reasonable mnemonics, or analogs in the shells.
58 Nevertheless, if you wish to use long variable names, you need only say:
62 at the top of your program. This aliases all the short names to the long
63 names in the current package. Some even have medium names, generally
64 borrowed from B<awk>. For more info, please see L<English>.
66 Before you continue, note the sort order for variables. In general, we
67 first list the variables in case-insensitive, almost-lexigraphical
68 order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
69 or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
70 For variables with the same identifier, we list it in order of scalar,
71 array, hash, and bareword.
73 =head2 General Variables
82 The default input and pattern-searching space. The following pairs are
85 while (<>) {...} # equivalent only in while!
86 while (defined($_ = <>)) {...}
97 Here are the places where Perl will assume C<$_> even if you don't use it:
103 The following functions use C<$_> as a default argument:
105 abs, alarm, chomp, chop, chr, chroot,
106 cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc,
107 lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf,
108 quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
109 rmdir, say, sin, split (for its second
110 argument), sqrt, stat, study, uc, ucfirst,
115 All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
120 The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
121 when used without an C<=~> operator.
125 The default iterator variable in a C<foreach> loop if no other
126 variable is supplied.
130 The implicit iterator variable in the C<grep()> and C<map()> functions.
134 The implicit variable of C<given()>.
138 The default place to put the next value or input record
139 when a C<< <FH> >>, C<readline>, C<readdir> or C<each>
140 operation's result is tested by itself as the sole criterion of a C<while>
141 test. Outside a C<while> test, this will not happen.
145 C<$_> is by default a global variable. However, as
146 of perl v5.10.0, you can use a lexical version of
147 C<$_> by declaring it in a file or in a block with C<my>. Moreover,
148 declaring C<our $_> restores the global C<$_> in the current scope. Though
149 this seemed like a good idea at the time it was introduced, lexical C<$_>
150 actually causes more problems than it solves. If you call a function that
151 expects to be passed information via C<$_>, it may or may not work,
152 depending on how the function is written, there not being any easy way to
153 solve this. Just avoid lexical C<$_>, unless you are feeling particularly
154 masochistic. For this reason lexical C<$_> is still experimental and will
155 produce a warning unless warnings have been disabled. As with other
156 experimental features, the behavior of lexical C<$_> is subject to change
157 without notice, including change into a fatal error.
159 Mnemonic: underline is understood in certain operations.
166 Within a subroutine the array C<@_> contains the parameters passed to
167 that subroutine. Inside a subroutine, C<@_> is the default array for
168 the array operators C<pop> and C<shift>.
172 =item $LIST_SEPARATOR
175 X<$"> X<$LIST_SEPARATOR>
177 When an array or an array slice is interpolated into a double-quoted
178 string or a similar context such as C</.../>, its elements are
179 separated by this value. Default is a space. For example, this:
181 print "The array is: @array\n";
183 is equivalent to this:
185 print "The array is: " . join($", @array) . "\n";
187 Mnemonic: works in double-quoted context.
194 X<$$> X<$PID> X<$PROCESS_ID>
196 The process number of the Perl running this script. Though you I<can> set
197 this variable, doing so is generally discouraged, although it can be
198 invaluable for some testing purposes. It will be reset automatically
199 across C<fork()> calls.
201 Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl
202 would emulate POSIX semantics on Linux systems using LinuxThreads, a
203 partial implementation of POSIX Threads that has since been superseded
204 by the Native POSIX Thread Library (NPTL).
206 LinuxThreads is now obsolete on Linux, and caching C<getpid()>
207 like this made embedding perl unnecessarily complex (since you'd have
208 to manually update the value of $$), so now C<$$> and C<getppid()>
209 will always return the same values as the underlying C library.
211 Debian GNU/kFreeBSD systems also used LinuxThreads up until and
212 including the 6.0 release, but after that moved to FreeBSD thread
213 semantics, which are POSIX-like.
215 To see if your system is affected by this discrepancy check if
216 C<getconf GNU_LIBPTHREAD_VERSION | grep -q NPTL> returns a false
217 value. NTPL threads preserve the POSIX semantics.
219 Mnemonic: same as shells.
224 X<$0> X<$PROGRAM_NAME>
226 Contains the name of the program being executed.
228 On some (but not all) operating systems assigning to C<$0> modifies
229 the argument area that the C<ps> program sees. On some platforms you
230 may have to use special C<ps> options or a different C<ps> to see the
231 changes. Modifying the C<$0> is more useful as a way of indicating the
232 current program state than it is for hiding the program you're
235 Note that there are platform-specific limitations on the maximum
236 length of C<$0>. In the most extreme case it may be limited to the
237 space occupied by the original C<$0>.
239 In some platforms there may be arbitrary amount of padding, for
240 example space characters, after the modified name as shown by C<ps>.
241 In some platforms this padding may extend all the way to the original
242 length of the argument area, no matter what you do (this is the case
243 for example with Linux 2.2).
245 Note for BSD users: setting C<$0> does not completely remove "perl"
246 from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
247 result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
248 and the " (perl)" suffix are shown depends on your exact BSD variant
249 and version). This is an operating system feature, Perl cannot help it.
251 In multithreaded scripts Perl coordinates the threads so that any
252 thread may modify its copy of the C<$0> and the change becomes visible
253 to ps(1) (assuming the operating system plays along). Note that
254 the view of C<$0> the other threads have will not change since they
255 have their own copies of it.
257 If the program has been given to perl via the switches C<-e> or C<-E>,
258 C<$0> will contain the string C<"-e">.
260 On Linux as of perl v5.14.0 the legacy process name will be set with
261 C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
262 perl has done since version 4.000. Now system utilities that read the
263 legacy process name such as ps, top and killall will recognize the
264 name you set when assigning to C<$0>. The string you supply will be
265 cut off at 16 bytes, this is a limitation imposed by Linux.
267 Mnemonic: same as B<sh> and B<ksh>.
274 X<$(> X<$GID> X<$REAL_GROUP_ID>
276 The real gid of this process. If you are on a machine that supports
277 membership in multiple groups simultaneously, gives a space separated
278 list of groups you are in. The first number is the one returned by
279 C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
280 the same as the first number.
282 However, a value assigned to C<$(> must be a single number used to
283 set the real gid. So the value given by C<$(> should I<not> be assigned
284 back to C<$(> without being forced numeric, such as by adding zero. Note
285 that this is different to the effective gid (C<$)>) which does take a
288 You can change both the real gid and the effective gid at the same
289 time by using C<POSIX::setgid()>. Changes
290 to C<$(> require a check to C<$!>
291 to detect any possible errors after an attempted change.
293 Mnemonic: parentheses are used to I<group> things. The real gid is the
294 group you I<left>, if you're running setgid.
296 =item $EFFECTIVE_GROUP_ID
301 X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
303 The effective gid of this process. If you are on a machine that
304 supports membership in multiple groups simultaneously, gives a space
305 separated list of groups you are in. The first number is the one
306 returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
307 one of which may be the same as the first number.
309 Similarly, a value assigned to C<$)> must also be a space-separated
310 list of numbers. The first number sets the effective gid, and
311 the rest (if any) are passed to C<setgroups()>. To get the effect of an
312 empty list for C<setgroups()>, just repeat the new effective gid; that is,
313 to force an effective gid of 5 and an effectively empty C<setgroups()>
314 list, say C< $) = "5 5" >.
316 You can change both the effective gid and the real gid at the same
317 time by using C<POSIX::setgid()> (use only a single numeric argument).
318 Changes to C<$)> require a check to C<$!> to detect any possible errors
319 after an attempted change.
321 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
322 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
323 and C<$)> can be swapped only on machines supporting C<setregid()>.
325 Mnemonic: parentheses are used to I<group> things. The effective gid
326 is the group that's I<right> for you, if you're running setgid.
333 X<< $< >> X<$UID> X<$REAL_USER_ID>
335 The real uid of this process. You can change both the real uid and the
336 effective uid at the same time by using C<POSIX::setuid()>. Since
337 changes to C<< $< >> require a system call, check C<$!> after a change
338 attempt to detect any possible errors.
340 Mnemonic: it's the uid you came I<from>, if you're running setuid.
342 =item $EFFECTIVE_USER_ID
347 X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
349 The effective uid of this process. For example:
351 $< = $>; # set real to effective uid
352 ($<,$>) = ($>,$<); # swap real and effective uids
354 You can change both the effective uid and the real uid at the same
355 time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
356 to C<$!> to detect any possible errors after an attempted change.
358 C<< $< >> and C<< $> >> can be swapped only on machines
359 supporting C<setreuid()>.
361 Mnemonic: it's the uid you went I<to>, if you're running setuid.
363 =item $SUBSCRIPT_SEPARATOR
368 X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
370 The subscript separator for multidimensional array emulation. If you
371 refer to a hash element as
377 $foo{join($;, $x, $y, $z)}
381 @foo{$x,$y,$z} # a slice--note the @
385 ($foo{$x},$foo{$y},$foo{$z})
387 Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
388 binary data there might not be any safe value for C<$;>.
390 Consider using "real" multidimensional arrays as described
393 Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
400 Special package variables when using C<sort()>, see L<perlfunc/sort>.
401 Because of this specialness C<$a> and C<$b> don't need to be declared
402 (using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
403 pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
404 be able to use them in the C<sort()> comparison block or function.
409 The hash C<%ENV> contains your current environment. Setting a
410 value in C<ENV> changes the environment for any child processes
411 you subsequently C<fork()> off.
413 As of v5.18.0, both keys and values stored in C<%ENV> are stringified.
417 if( ref $ENV{'bar'} ) {
418 say "Pre 5.18.0 Behaviour";
420 say "Post 5.18.0 Behaviour";
423 Previously, only child processes received stringified values:
428 # Always printed 'non ref'
430 q/print ( ref $ENV{'bar'} ? 'ref' : 'non ref' ) /);
432 This happens because you can't really share arbitrary data structures with
435 =item $OLD_PERL_VERSION
438 X<$]> X<$OLD_PERL_VERSION>
440 The revision, version, and subversion of the Perl interpreter, represented
441 as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY
442 is the subversion / 1e6. For example, Perl v5.10.1 would be "5.010001".
444 This variable can be used to determine whether the Perl interpreter
445 executing a script is in the right range of versions:
447 warn "No PerlIO!\n" if $] lt '5.008';
449 When comparing C<$]>, string comparison operators are B<highly
450 recommended>. The inherent limitations of binary floating point
451 representation can sometimes lead to incorrect comparisons for some
452 numbers on some architectures.
454 See also the documentation of C<use VERSION> and C<require VERSION>
455 for a convenient way to fail if the running Perl interpreter is too old.
457 See L</$^V> for a representation of the Perl version as a L<version>
458 object, which allows more flexible string comparisons.
460 The main advantage of C<$]> over C<$^V> is that it works the same on any
461 version of Perl. The disadvantages are that it can't easily be compared
462 to versions in other formats (e.g. literal v-strings, "v1.2.3" or
463 version objects) and numeric comparisons can occasionally fail; it's good
464 for string literal version checks and bad for comparing to a variable
465 that hasn't been sanity-checked.
467 The C<$OLD_PERL_VERSION> form was added in Perl v5.20.0 for historical
468 reasons but its use is discouraged. (If your reason to use C<$]> is to
469 run code on old perls then referring to it as C<$OLD_PERL_VERSION> would
472 Mnemonic: Is this version of perl in the right bracket?
477 X<$^F> X<$SYSTEM_FD_MAX>
479 The maximum system file descriptor, ordinarily 2. System file
480 descriptors are passed to C<exec()>ed processes, while higher file
481 descriptors are not. Also, during an
482 C<open()>, system file descriptors are
483 preserved even if the C<open()> fails (ordinary file descriptors are
484 closed before the C<open()> is attempted). The close-on-exec
485 status of a file descriptor will be decided according to the value of
486 C<$^F> when the corresponding file, pipe, or socket was opened, not the
487 time of the C<exec()>.
492 The array C<@F> contains the fields of each line read in when autosplit
493 mode is turned on. See L<perlrun> for the B<-a> switch. This array
494 is package-specific, and must be declared or given a full package name
495 if not in package main when running under C<strict 'vars'>.
500 The array C<@INC> contains the list of places that the C<do EXPR>,
501 C<require>, or C<use> constructs look for their library files. It
502 initially consists of the arguments to any B<-I> command-line
503 switches, followed by the default Perl library, probably
504 F</usr/local/lib/perl>, followed by ".", to represent the current
505 directory. ("." will not be appended if taint checks are enabled,
506 either by C<-T> or by C<-t>.) If you need to modify this at runtime,
507 you should use the C<use lib> pragma to get the machine-dependent
508 library properly loaded also:
510 use lib '/mypath/libdir/';
513 You can also insert hooks into the file inclusion system by putting Perl
514 code directly into C<@INC>. Those hooks may be subroutine references,
515 array references or blessed objects. See L<perlfunc/require> for details.
520 The hash C<%INC> contains entries for each filename included via the
521 C<do>, C<require>, or C<use> operators. The key is the filename
522 you specified (with module names converted to pathnames), and the
523 value is the location of the file found. The C<require>
524 operator uses this hash to determine whether a particular file has
525 already been included.
527 If the file was loaded via a hook (e.g. a subroutine reference, see
528 L<perlfunc/require> for a description of these hooks), this hook is
529 by default inserted into C<%INC> in place of a filename. Note, however,
530 that the hook may have set the C<%INC> entry by itself to provide some more
536 X<$^I> X<$INPLACE_EDIT>
538 The current value of the inplace-edit extension. Use C<undef> to disable
541 Mnemonic: value of B<-i> switch.
546 By default, running out of memory is an untrappable, fatal error.
547 However, if suitably built, Perl can use the contents of C<$^M>
548 as an emergency memory pool after C<die()>ing. Suppose that your Perl
549 were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
552 $^M = 'a' x (1 << 16);
554 would allocate a 64K buffer for use in an emergency. See the
555 F<INSTALL> file in the Perl distribution for information on how to
556 add custom C compilation flags when compiling perl. To discourage casual
557 use of this advanced feature, there is no L<English|English> long name for
560 This variable was added in Perl 5.004.
567 The name of the operating system under which this copy of Perl was
568 built, as determined during the configuration process. For examples
569 see L<perlport/PLATFORMS>.
571 The value is identical to C<$Config{'osname'}>. See also L<Config>
572 and the B<-V> command-line switch documented in L<perlrun>.
574 In Windows platforms, C<$^O> is not very helpful: since it is always
575 C<MSWin32>, it doesn't tell the difference between
576 95/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
577 Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
578 between the variants.
580 This variable was added in Perl 5.003.
585 The hash C<%SIG> contains signal handlers for signals. For example:
587 sub handler { # 1st argument is signal name
589 print "Caught a SIG$sig--shutting down\n";
594 $SIG{'INT'} = \&handler;
595 $SIG{'QUIT'} = \&handler;
597 $SIG{'INT'} = 'DEFAULT'; # restore default action
598 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
600 Using a value of C<'IGNORE'> usually has the effect of ignoring the
601 signal, except for the C<CHLD> signal. See L<perlipc> for more about
604 Here are some other examples:
606 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not
608 $SIG{"PIPE"} = \&Plumber; # just fine; assume current
610 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
611 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber()
614 Be sure not to use a bareword as the name of a signal handler,
615 lest you inadvertently call it.
617 If your system has the C<sigaction()> function then signal handlers
618 are installed using it. This means you get reliable signal handling.
620 The default delivery policy of signals changed in Perl v5.8.0 from
621 immediate (also known as "unsafe") to deferred, also known as "safe
622 signals". See L<perlipc> for more information.
624 Certain internal hooks can be also set using the C<%SIG> hash. The
625 routine indicated by C<$SIG{__WARN__}> is called when a warning
626 message is about to be printed. The warning message is passed as the
627 first argument. The presence of a C<__WARN__> hook causes the
628 ordinary printing of warnings to C<STDERR> to be suppressed. You can
629 use this to save warnings in a variable, or turn warnings into fatal
632 local $SIG{__WARN__} = sub { die $_[0] };
635 As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
636 disable warnings using the empty subroutine:
638 local $SIG{__WARN__} = sub {};
640 The routine indicated by C<$SIG{__DIE__}> is called when a fatal
641 exception is about to be thrown. The error message is passed as the
642 first argument. When a C<__DIE__> hook routine returns, the exception
643 processing continues as it would have in the absence of the hook,
644 unless the hook routine itself exits via a C<goto &sub>, a loop exit,
645 or a C<die()>. The C<__DIE__> handler is explicitly disabled during
646 the call, so that you can die from a C<__DIE__> handler. Similarly
649 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
650 even inside an C<eval()>. Do not use this to rewrite a pending
651 exception in C<$@>, or as a bizarre substitute for overriding
652 C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
653 in a future release so that C<$SIG{__DIE__}> is only called if your
654 program is about to exit, as was the original intent. Any other use is
657 C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
658 may be called to report (probable) errors found by the parser. In such
659 a case the parser may be in inconsistent state, so any attempt to
660 evaluate Perl code from such a handler will probably result in a
661 segfault. This means that warnings or errors that result from parsing
662 Perl should be used with extreme caution, like this:
664 require Carp if defined $^S;
665 Carp::confess("Something wrong") if defined &Carp::confess;
666 die "Something wrong, but could not load Carp to give "
668 . "To see backtrace try starting Perl with -MCarp switch";
670 Here the first line will load C<Carp> I<unless> it is the parser who
671 called the handler. The second line will print backtrace and die if
672 C<Carp> was available. The third line will be executed only if C<Carp> was
675 Having to even think about the C<$^S> variable in your exception
676 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
677 invites grievous and difficult to track down errors. Avoid it
678 and use an C<END{}> or CORE::GLOBAL::die override instead.
680 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
681 L<warnings> for additional information.
688 The time at which the program began running, in seconds since the
689 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
690 and B<-C> filetests are based on this value.
695 X<$^V> X<$PERL_VERSION>
697 The revision, version, and subversion of the Perl interpreter,
698 represented as a L<version> object.
700 This variable first appeared in perl v5.6.0; earlier versions of perl
701 will see an undefined value. Before perl v5.10.0 C<$^V> was represented
702 as a v-string rather than a L<version> object.
704 C<$^V> can be used to determine whether the Perl interpreter executing
705 a script is in the right range of versions. For example:
707 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
709 While version objects overload stringification, to portably convert
710 C<$^V> into its string representation, use C<sprintf()>'s C<"%vd">
711 conversion, which works for both v-strings or version objects:
713 printf "version is v%vd\n", $^V; # Perl's version
715 See the documentation of C<use VERSION> and C<require VERSION>
716 for a convenient way to fail if the running Perl interpreter is too old.
718 See also C<L</$]>> for a decimal representation of the Perl version.
720 The main advantage of C<$^V> over C<$]> is that, for Perl v5.10.0 or
721 later, it overloads operators, allowing easy comparison against other
722 version representations (e.g. decimal, literal v-string, "v1.2.3", or
723 objects). The disadvantage is that prior to v5.10.0, it was only a
724 literal v-string, which can't be easily printed or compared, whereas
725 the behavior of C<$]> is unchanged on all versions of Perl.
727 Mnemonic: use ^V for a version object.
729 =item ${^WIN32_SLOPPY_STAT}
730 X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
732 If this variable is set to a true value, then C<stat()> on Windows will
733 not try to open the file. This means that the link count cannot be
734 determined and file attributes may be out of date if additional
735 hardlinks to the file exist. On the other hand, not opening the file
736 is considerably faster, especially for files on network drives.
738 This variable could be set in the F<sitecustomize.pl> file to
739 configure the local Perl installation to use "sloppy" C<stat()> by
740 default. See the documentation for B<-f> in
741 L<perlrun|perlrun/"Command Switches"> for more information about site
744 This variable was added in Perl v5.10.0.
746 =item $EXECUTABLE_NAME
749 X<$^X> X<$EXECUTABLE_NAME>
751 The name used to execute the current copy of Perl, from C's
752 C<argv[0]> or (where supported) F</proc/self/exe>.
754 Depending on the host operating system, the value of C<$^X> may be
755 a relative or absolute pathname of the perl program file, or may
756 be the string used to invoke perl but not the pathname of the
757 perl program file. Also, most operating systems permit invoking
758 programs that are not in the PATH environment variable, so there
759 is no guarantee that the value of C<$^X> is in PATH. For VMS, the
760 value may or may not include a version number.
762 You usually can use the value of C<$^X> to re-invoke an independent
763 copy of the same perl that is currently running, e.g.,
765 @first_run = `$^X -le "print int rand 100 for 1..100"`;
767 But recall that not all operating systems support forking or
768 capturing of the output of commands, so this complex statement
771 It is not safe to use the value of C<$^X> as a path name of a file,
772 as some operating systems that have a mandatory suffix on
773 executable files do not require use of the suffix when invoking
774 a command. To convert the value of C<$^X> to a path name, use the
775 following statements:
777 # Build up a set of file names (not command names).
781 $this_perl .= $Config{_exe}
782 unless $this_perl =~ m/$Config{_exe}$/i;
785 Because many operating systems permit anyone with read access to
786 the Perl program file to make a copy of it, patch the copy, and
787 then execute the copy, the security-conscious Perl programmer
788 should take care to invoke the installed copy of perl, not the
789 copy referenced by C<$^X>. The following statements accomplish
790 this goal, and produce a pathname that can be invoked as a
791 command or referenced as a file.
794 my $secure_perl_path = $Config{perlpath};
796 $secure_perl_path .= $Config{_exe}
797 unless $secure_perl_path =~ m/$Config{_exe}$/i;
802 =head2 Variables related to regular expressions
804 Most of the special variables related to regular expressions are side
805 effects. Perl sets these variables when it has a successful match, so
806 you should check the match result before using them. For instance:
808 if( /P(A)TT(ER)N/ ) {
809 print "I found $1 and $2\n";
812 These variables are read-only and dynamically-scoped, unless we note
815 The dynamic nature of the regular expression variables means that
816 their value is limited to the block that they are in, as demonstrated
819 my $outer = 'Wallace and Grommit';
820 my $inner = 'Mutt and Jeff';
822 my $pattern = qr/(\S+) and (\S+)/;
824 sub show_n { print "\$1 is $1; \$2 is $2\n" }
828 show_n() if $outer =~ m/$pattern/;
831 show_n() if $inner =~ m/$pattern/;
837 The output shows that while in the C<OUTER> block, the values of C<$1>
838 and C<$2> are from the match against C<$outer>. Inside the C<INNER>
839 block, the values of C<$1> and C<$2> are from the match against
840 C<$inner>, but only until the end of the block (i.e. the dynamic
841 scope). After the C<INNER> block completes, the values of C<$1> and
842 C<$2> return to the values for the match against C<$outer> even though
843 we have not made another match:
845 $1 is Wallace; $2 is Grommit
846 $1 is Mutt; $2 is Jeff
847 $1 is Wallace; $2 is Grommit
849 =head3 Performance issues
851 Traditionally in Perl, any use of any of the three variables C<$`>, C<$&>
852 or C<$'> (or their C<use English> equivalents) anywhere in the code, caused
853 all subsequent successful pattern matches to make a copy of the matched
854 string, in case the code might subsequently access one of those variables.
855 This imposed a considerable performance penalty across the whole program,
856 so generally the use of these variables has been discouraged.
858 In Perl 5.6.0 the C<@-> and C<@+> dynamic arrays were introduced that
859 supply the indices of successful matches. So you could for example do
864 print $`, $&, $'; # bad: perfomance hit
866 print # good: no perfomance hit
867 substr($str, 0, $-[0]),
868 substr($str, $-[0], $+[0]-$-[0]),
871 In Perl 5.10.0 the C</p> match operator flag and the C<${^PREMATCH}>,
872 C<${^MATCH}>, and C<${^POSTMATCH}> variables were introduced, that allowed
873 you to suffer the penalties only on patterns marked with C</p>.
875 In Perl 5.18.0 onwards, perl started noting the presence of each of the
876 three variables separately, and only copied that part of the string
879 $`; $&; "abcdefgh" =~ /d/
881 perl would only copy the "abcd" part of the string. That could make a big
882 difference in something like
884 $str = 'x' x 1_000_000;
886 $str =~ /x/g # one char copied a million times, not a million chars
888 In Perl 5.20.0 a new copy-on-write system was enabled by default, which
889 finally fixes all performance issues with these three variables, and makes
890 them safe to use anywhere.
892 The C<Devel::NYTProf> and C<Devel::FindAmpersand> modules can help you
893 find uses of these problematic match variables in your code.
897 =item $<I<digits>> ($1, $2, ...)
900 Contains the subpattern from the corresponding set of capturing
901 parentheses from the last successful pattern match, not counting patterns
902 matched in nested blocks that have been exited already.
904 These variables are read-only and dynamically-scoped.
906 Mnemonic: like \digits.
913 The string matched by the last successful pattern match (not counting
914 any matches hidden within a BLOCK or C<eval()> enclosed by the current
917 See L</Performance issues> above for the serious performance implications
918 of using this variable (even once) in your code.
920 This variable is read-only and dynamically-scoped.
922 Mnemonic: like C<&> in some editors.
927 This is similar to C<$&> (C<$MATCH>) except that it does not incur the
928 performance penalty associated with that variable.
930 See L</Performance issues> above.
932 In Perl v5.18 and earlier, it is only guaranteed
933 to return a defined value when the pattern was compiled or executed with
934 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
935 C<${^MATCH}> does the same thing as C<$MATCH>.
937 This variable was added in Perl v5.10.0.
939 This variable is read-only and dynamically-scoped.
944 X<$`> X<$PREMATCH> X<${^PREMATCH}>
946 The string preceding whatever was matched by the last successful
947 pattern match, not counting any matches hidden within a BLOCK or C<eval>
948 enclosed by the current BLOCK.
950 See L</Performance issues> above for the serious performance implications
951 of using this variable (even once) in your code.
953 This variable is read-only and dynamically-scoped.
955 Mnemonic: C<`> often precedes a quoted string.
958 X<$`> X<${^PREMATCH}>
960 This is similar to C<$`> ($PREMATCH) except that it does not incur the
961 performance penalty associated with that variable.
963 See L</Performance issues> above.
965 In Perl v5.18 and earlier, it is only guaranteed
966 to return a defined value when the pattern was compiled or executed with
967 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
968 C<${^PREMATCH}> does the same thing as C<$PREMATCH>.
970 This variable was added in Perl v5.10.0.
972 This variable is read-only and dynamically-scoped.
977 X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
979 The string following whatever was matched by the last successful
980 pattern match (not counting any matches hidden within a BLOCK or C<eval()>
981 enclosed by the current BLOCK). Example:
983 local $_ = 'abcdefghi';
985 print "$`:$&:$'\n"; # prints abc:def:ghi
987 See L</Performance issues> above for the serious performance implications
988 of using this variable (even once) in your code.
990 This variable is read-only and dynamically-scoped.
992 Mnemonic: C<'> often follows a quoted string.
995 X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
997 This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
998 performance penalty associated with that variable.
1000 See L</Performance issues> above.
1002 In Perl v5.18 and earlier, it is only guaranteed
1003 to return a defined value when the pattern was compiled or executed with
1004 the C</p> modifier. In Perl v5.20, the C</p> modifier does nothing, so
1005 C<${^POSTMATCH}> does the same thing as C<$POSTMATCH>.
1007 This variable was added in Perl v5.10.0.
1009 This variable is read-only and dynamically-scoped.
1011 =item $LAST_PAREN_MATCH
1014 X<$+> X<$LAST_PAREN_MATCH>
1016 The text matched by the last bracket of the last successful search pattern.
1017 This is useful if you don't know which one of a set of alternative patterns
1018 matched. For example:
1020 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
1022 This variable is read-only and dynamically-scoped.
1024 Mnemonic: be positive and forward looking.
1026 =item $LAST_SUBMATCH_RESULT
1029 X<$^N> X<$LAST_SUBMATCH_RESULT>
1031 The text matched by the used group most-recently closed (i.e. the group
1032 with the rightmost closing parenthesis) of the last successful search
1035 This is primarily used inside C<(?{...})> blocks for examining text
1036 recently matched. For example, to effectively capture text to a variable
1037 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
1039 (?:(...)(?{ $var = $^N }))
1041 By setting and then using C<$var> in this way relieves you from having to
1042 worry about exactly which numbered set of parentheses they are.
1044 This variable was added in Perl v5.8.0.
1046 Mnemonic: the (possibly) Nested parenthesis that most recently closed.
1048 =item @LAST_MATCH_END
1051 X<@+> X<@LAST_MATCH_END>
1053 This array holds the offsets of the ends of the last successful
1054 submatches in the currently active dynamic scope. C<$+[0]> is
1055 the offset into the string of the end of the entire match. This
1056 is the same value as what the C<pos> function returns when called
1057 on the variable that was matched against. The I<n>th element
1058 of this array holds the offset of the I<n>th submatch, so
1059 C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
1060 past where C<$2> ends, and so on. You can use C<$#+> to determine
1061 how many subgroups were in the last successful match. See the
1062 examples given for the C<@-> variable.
1064 This variable was added in Perl v5.6.0.
1066 =item %LAST_PAREN_MATCH
1069 X<%+> X<%LAST_PAREN_MATCH>
1071 Similar to C<@+>, the C<%+> hash allows access to the named capture
1072 buffers, should they exist, in the last successful match in the
1073 currently active dynamic scope.
1075 For example, C<$+{foo}> is equivalent to C<$1> after the following match:
1077 'foo' =~ /(?<foo>foo)/;
1079 The keys of the C<%+> hash list only the names of buffers that have
1080 captured (and that are thus associated to defined values).
1082 The underlying behaviour of C<%+> is provided by the
1083 L<Tie::Hash::NamedCapture> module.
1085 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1086 associated with the last successful regular expression. Therefore mixing
1087 iterative access to them via C<each> may have unpredictable results.
1088 Likewise, if the last successful match changes, then the results may be
1091 This variable was added in Perl v5.10.0.
1093 This variable is read-only and dynamically-scoped.
1095 =item @LAST_MATCH_START
1098 X<@-> X<@LAST_MATCH_START>
1100 C<$-[0]> is the offset of the start of the last successful match.
1101 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
1102 I<n>-th subpattern, or undef if the subpattern did not match.
1104 Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
1105 $+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
1106 $+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
1107 C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the
1108 last matched subgroup in the last successful match. Contrast with
1109 C<$#+>, the number of subgroups in the regular expression. Compare
1112 This array holds the offsets of the beginnings of the last
1113 successful submatches in the currently active dynamic scope.
1114 C<$-[0]> is the offset into the string of the beginning of the
1115 entire match. The I<n>th element of this array holds the offset
1116 of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
1117 begins, C<$-[2]> the offset where C<$2> begins, and so on.
1119 After a match against some variable C<$var>:
1123 =item C<$`> is the same as C<substr($var, 0, $-[0])>
1125 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1127 =item C<$'> is the same as C<substr($var, $+[0])>
1129 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1131 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1133 =item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1137 This variable was added in Perl v5.6.0.
1142 Similar to C<%+>, this variable allows access to the named capture groups
1143 in the last successful match in the currently active dynamic scope. To
1144 each capture group name found in the regular expression, it associates a
1145 reference to an array containing the list of values captured by all
1146 buffers with that name (should there be several of them), in the order
1151 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1152 foreach my $bufname (sort keys %-) {
1153 my $ary = $-{$bufname};
1154 foreach my $idx (0..$#$ary) {
1155 print "\$-{$bufname}[$idx] : ",
1156 (defined($ary->[$idx])
1171 The keys of the C<%-> hash correspond to all buffer names found in
1172 the regular expression.
1174 The behaviour of C<%-> is implemented via the
1175 L<Tie::Hash::NamedCapture> module.
1177 B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1178 associated with the last successful regular expression. Therefore mixing
1179 iterative access to them via C<each> may have unpredictable results.
1180 Likewise, if the last successful match changes, then the results may be
1183 This variable was added in Perl v5.10.0.
1185 This variable is read-only and dynamically-scoped.
1187 =item $LAST_REGEXP_CODE_RESULT
1190 X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1192 The result of evaluation of the last successful C<(?{ code })>
1193 regular expression assertion (see L<perlre>). May be written to.
1195 This variable was added in Perl 5.005.
1197 =item ${^RE_DEBUG_FLAGS}
1198 X<${^RE_DEBUG_FLAGS}>
1200 The current value of the regex debugging flags. Set to 0 for no debug output
1201 even when the C<re 'debug'> module is loaded. See L<re> for details.
1203 This variable was added in Perl v5.10.0.
1205 =item ${^RE_TRIE_MAXBUF}
1206 X<${^RE_TRIE_MAXBUF}>
1208 Controls how certain regex optimisations are applied and how much memory they
1209 utilize. This value by default is 65536 which corresponds to a 512kB
1210 temporary cache. Set this to a higher value to trade
1211 memory for speed when matching large alternations. Set
1212 it to a lower value if you want the optimisations to
1213 be as conservative of memory as possible but still occur, and set it to a
1214 negative value to prevent the optimisation and conserve the most memory.
1215 Under normal situations this variable should be of no interest to you.
1217 This variable was added in Perl v5.10.0.
1221 =head2 Variables related to filehandles
1223 Variables that depend on the currently selected filehandle may be set
1224 by calling an appropriate object method on the C<IO::Handle> object,
1225 although this is less efficient than using the regular built-in
1226 variables. (Summary lines below for this contain the word HANDLE.)
1231 after which you may use either
1237 HANDLE->method(EXPR)
1239 Each method returns the old value of the C<IO::Handle> attribute. The
1240 methods each take an optional EXPR, which, if supplied, specifies the
1241 new value for the C<IO::Handle> attribute in question. If not
1242 supplied, most methods do nothing to the current value--except for
1243 C<autoflush()>, which will assume a 1 for you, just to be different.
1245 Because loading in the C<IO::Handle> class is an expensive operation,
1246 you should learn how to use the regular built-in variables.
1248 A few of these variables are considered "read-only". This means that
1249 if you try to assign to this variable, either directly or indirectly
1250 through a reference, you'll raise a run-time exception.
1252 You should be very careful when modifying the default values of most
1253 special variables described in this document. In most cases you want
1254 to localize these variables before changing them, since if you don't,
1255 the change may affect other modules which rely on the default values
1256 of the special variables that you have changed. This is one of the
1257 correct ways to read the whole file at once:
1259 open my $fh, "<", "foo" or die $!;
1260 local $/; # enable localized slurp mode
1261 my $content = <$fh>;
1264 But the following code is quite bad:
1266 open my $fh, "<", "foo" or die $!;
1267 undef $/; # enable slurp mode
1268 my $content = <$fh>;
1271 since some other module, may want to read data from some file in the
1272 default "line mode", so if the code we have just presented has been
1273 executed, the global value of C<$/> is now changed for any other code
1274 running inside the same Perl interpreter.
1276 Usually when a variable is localized you want to make sure that this
1277 change affects the shortest scope possible. So unless you are already
1278 inside some short C<{}> block, you should create one yourself. For
1282 open my $fh, "<", "foo" or die $!;
1289 Here is an example of how your own code can go broken:
1299 # do something with $_
1302 You probably expect this code to print the equivalent of
1306 but instead you get:
1310 Why? Because C<nasty_break()> modifies C<$\> without localizing it
1311 first. The value you set in C<nasty_break()> is still there when you
1312 return. The fix is to add C<local()> so the value doesn't leak out of
1317 It's easy to notice the problem in such a short example, but in more
1318 complicated code you are looking for trouble if you don't localize
1319 changes to the special variables.
1326 Contains the name of the current file when reading from C<< <> >>.
1331 The array C<@ARGV> contains the command-line arguments intended for
1332 the script. C<$#ARGV> is generally the number of arguments minus
1333 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1334 command name itself. See L</$0> for the command name.
1339 The special filehandle that iterates over command-line filenames in
1340 C<@ARGV>. Usually written as the null filehandle in the angle operator
1341 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1342 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1343 corresponding to the last file opened by C<< <> >>. In particular,
1344 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1345 may not cause your function to automatically read the contents of all the
1351 The special filehandle that points to the currently open output file
1352 when doing edit-in-place processing with B<-i>. Useful when you have
1353 to do a lot of inserting and don't want to keep modifying C<$_>. See
1354 L<perlrun> for the B<-i> switch.
1356 =item IO::Handle->output_field_separator( EXPR )
1358 =item $OUTPUT_FIELD_SEPARATOR
1363 X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1365 The output field separator for the print operator. If defined, this
1366 value is printed between each of print's arguments. Default is C<undef>.
1368 You cannot call C<output_field_separator()> on a handle, only as a
1369 static method. See L<IO::Handle|IO::Handle>.
1371 Mnemonic: what is printed when there is a "," in your print statement.
1373 =item HANDLE->input_line_number( EXPR )
1375 =item $INPUT_LINE_NUMBER
1380 X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1382 Current line number for the last filehandle accessed.
1384 Each filehandle in Perl counts the number of lines that have been read
1385 from it. (Depending on the value of C<$/>, Perl's idea of what
1386 constitutes a line may not match yours.) When a line is read from a
1387 filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1388 C<seek()> is called on it, C<$.> becomes an alias to the line counter
1389 for that filehandle.
1391 You can adjust the counter by assigning to C<$.>, but this will not
1392 actually move the seek pointer. I<Localizing C<$.> will not localize
1393 the filehandle's line count>. Instead, it will localize perl's notion
1394 of which filehandle C<$.> is currently aliased to.
1396 C<$.> is reset when the filehandle is closed, but B<not> when an open
1397 filehandle is reopened without an intervening C<close()>. For more
1398 details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1399 an explicit close, line numbers increase across C<ARGV> files (but see
1400 examples in L<perlfunc/eof>).
1402 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1403 line counter for a given filehandle without having to worry about
1404 which handle you last accessed.
1406 Mnemonic: many programs use "." to mean the current line number.
1408 =item IO::Handle->input_record_separator( EXPR )
1410 =item $INPUT_RECORD_SEPARATOR
1415 X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1417 The input record separator, newline by default. This influences Perl's
1418 idea of what a "line" is. Works like B<awk>'s RS variable, including
1419 treating empty lines as a terminator if set to the null string (an
1420 empty line cannot contain any spaces or tabs). You may set it to a
1421 multi-character string to match a multi-character terminator, or to
1422 C<undef> to read through the end of file. Setting it to C<"\n\n">
1423 means something slightly different than setting to C<"">, if the file
1424 contains consecutive empty lines. Setting to C<""> will treat two or
1425 more consecutive empty lines as a single empty line. Setting to
1426 C<"\n\n"> will blindly assume that the next input character belongs to
1427 the next paragraph, even if it's a newline.
1429 local $/; # enable "slurp" mode
1430 local $_ = <FH>; # whole file now here
1433 Remember: the value of C<$/> is a string, not a regex. B<awk> has to
1434 be better for something. :-)
1436 Setting C<$/> to a reference to an integer, scalar containing an
1437 integer, or scalar that's convertible to an integer will attempt to
1438 read records instead of lines, with the maximum record size being the
1439 referenced integer number of characters. So this:
1441 local $/ = \32768; # or \"32768", or \$var_containing_32768
1442 open my $fh, "<", $myfile or die $!;
1445 will read a record of no more than 32768 characters from $fh. If you're
1446 not reading from a record-oriented file (or your OS doesn't have
1447 record-oriented files), then you'll likely get a full chunk of data
1448 with every read. If a record is larger than the record size you've
1449 set, you'll get the record back in pieces. Trying to set the record
1450 size to zero or less is deprecated and will cause $/ to have the value
1451 of "undef", which will cause reading in the (rest of the) whole file.
1453 As of 5.19.9 setting C<$/> to any other form of reference will throw a
1454 fatal exception. This is in preparation for supporting new ways to set
1455 C<$/> in the future.
1457 On VMS only, record reads bypass PerlIO layers and any associated
1458 buffering, so you must not mix record and non-record reads on the
1459 same filehandle. Record mode mixes with line mode only when the
1460 same buffering layer is in use for both modes.
1462 You cannot call C<input_record_separator()> on a handle, only as a
1463 static method. See L<IO::Handle|IO::Handle>.
1465 See also L<perlport/"Newlines">. Also see L</$.>.
1467 Mnemonic: / delimits line boundaries when quoting poetry.
1469 =item IO::Handle->output_record_separator( EXPR )
1471 =item $OUTPUT_RECORD_SEPARATOR
1476 X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
1478 The output record separator for the print operator. If defined, this
1479 value is printed after the last of print's arguments. Default is C<undef>.
1481 You cannot call C<output_record_separator()> on a handle, only as a
1482 static method. See L<IO::Handle|IO::Handle>.
1484 Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1485 Also, it's just like C<$/>, but it's what you get "back" from Perl.
1487 =item HANDLE->autoflush( EXPR )
1489 =item $OUTPUT_AUTOFLUSH
1492 X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
1494 If set to nonzero, forces a flush right away and after every write or
1495 print on the currently selected output channel. Default is 0
1496 (regardless of whether the channel is really buffered by the system or
1497 not; C<$|> tells you only whether you've asked Perl explicitly to
1498 flush after each write). STDOUT will typically be line buffered if
1499 output is to the terminal and block buffered otherwise. Setting this
1500 variable is useful primarily when you are outputting to a pipe or
1501 socket, such as when you are running a Perl program under B<rsh> and
1502 want to see the output as it's happening. This has no effect on input
1503 buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
1504 how to select the output channel. See also L<IO::Handle>.
1506 Mnemonic: when you want your pipes to be piping hot.
1511 This read-only variable contains a reference to the last-read filehandle.
1512 This is set by C<< <HANDLE> >>, C<readline>, C<tell>, C<eof> and C<seek>.
1513 This is the same handle that C<$.> and C<tell> and C<eof> without arguments
1514 use. It is also the handle used when Perl appends ", <STDIN> line 1" to
1515 an error or warning message.
1517 This variable was added in Perl v5.18.0.
1521 =head3 Variables related to formats
1523 The special variables for formats are a subset of those for
1524 filehandles. See L<perlform> for more information about Perl's
1532 X<$^A> X<$ACCUMULATOR>
1534 The current value of the C<write()> accumulator for C<format()> lines.
1535 A format contains C<formline()> calls that put their result into
1536 C<$^A>. After calling its format, C<write()> prints out the contents
1537 of C<$^A> and empties. So you never really see the contents of C<$^A>
1538 unless you call C<formline()> yourself and then look at it. See
1539 L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
1541 =item IO::Handle->format_formfeed(EXPR)
1543 =item $FORMAT_FORMFEED
1546 X<$^L> X<$FORMAT_FORMFEED>
1548 What formats output as a form feed. The default is C<\f>.
1550 You cannot call C<format_formfeed()> on a handle, only as a static
1551 method. See L<IO::Handle|IO::Handle>.
1553 =item HANDLE->format_page_number(EXPR)
1555 =item $FORMAT_PAGE_NUMBER
1558 X<$%> X<$FORMAT_PAGE_NUMBER>
1560 The current page number of the currently selected output channel.
1562 Mnemonic: C<%> is page number in B<nroff>.
1564 =item HANDLE->format_lines_left(EXPR)
1566 =item $FORMAT_LINES_LEFT
1569 X<$-> X<$FORMAT_LINES_LEFT>
1571 The number of lines left on the page of the currently selected output
1574 Mnemonic: lines_on_page - lines_printed.
1576 =item IO::Handle->format_line_break_characters EXPR
1578 =item $FORMAT_LINE_BREAK_CHARACTERS
1581 X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
1583 The current set of characters after which a string may be broken to
1584 fill continuation fields (starting with C<^>) in a format. The default is
1585 S<" \n-">, to break on a space, newline, or a hyphen.
1587 You cannot call C<format_line_break_characters()> on a handle, only as
1588 a static method. See L<IO::Handle|IO::Handle>.
1590 Mnemonic: a "colon" in poetry is a part of a line.
1592 =item HANDLE->format_lines_per_page(EXPR)
1594 =item $FORMAT_LINES_PER_PAGE
1597 X<$=> X<$FORMAT_LINES_PER_PAGE>
1599 The current page length (printable lines) of the currently selected
1600 output channel. The default is 60.
1602 Mnemonic: = has horizontal lines.
1604 =item HANDLE->format_top_name(EXPR)
1606 =item $FORMAT_TOP_NAME
1609 X<$^> X<$FORMAT_TOP_NAME>
1611 The name of the current top-of-page format for the currently selected
1612 output channel. The default is the name of the filehandle with C<_TOP>
1613 appended. For example, the default format top name for the C<STDOUT>
1614 filehandle is C<STDOUT_TOP>.
1616 Mnemonic: points to top of page.
1618 =item HANDLE->format_name(EXPR)
1623 X<$~> X<$FORMAT_NAME>
1625 The name of the current report format for the currently selected
1626 output channel. The default format name is the same as the filehandle
1627 name. For example, the default format name for the C<STDOUT>
1628 filehandle is just C<STDOUT>.
1630 Mnemonic: brother to C<$^>.
1634 =head2 Error Variables
1635 X<error> X<exception>
1637 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1638 about different types of error conditions that may appear during
1639 execution of a Perl program. The variables are shown ordered by
1640 the "distance" between the subsystem which reported the error and
1641 the Perl process. They correspond to errors detected by the Perl
1642 interpreter, C library, operating system, or an external program,
1645 To illustrate the differences between these variables, consider the
1646 following Perl expression, which uses a single-quoted string. After
1647 execution of this statement, perl may have set all four special error
1651 open my $pipe, "/cdrom/install |" or die $!;
1653 close $pipe or die "bad pipe: $?, $!";
1656 When perl executes the C<eval()> expression, it translates the
1657 C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
1658 and thence to the operating system kernel. perl sets C<$!> to
1659 the C library's C<errno> if one of these calls fails.
1661 C<$@> is set if the string to be C<eval>-ed did not compile (this may
1662 happen if C<open> or C<close> were imported with bad prototypes), or
1663 if Perl code executed during evaluation C<die()>d. In these cases the
1664 value of C<$@> is the compile error, or the argument to C<die> (which
1665 will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
1667 Under a few operating systems, C<$^E> may contain a more verbose error
1668 indicator, such as in this case, "CDROM tray not closed." Systems that
1669 do not support extended error messages leave C<$^E> the same as C<$!>.
1671 Finally, C<$?> may be set to a non-0 value if the external program
1672 F</cdrom/install> fails. The upper eight bits reflect specific error
1673 conditions encountered by the program (the program's C<exit()> value).
1674 The lower eight bits reflect mode of failure, like signal death and
1675 core dump information. See L<wait(2)> for details. In contrast to
1676 C<$!> and C<$^E>, which are set only if an error condition is detected,
1677 the variable C<$?> is set on each C<wait> or pipe C<close>,
1678 overwriting the old value. This is more like C<$@>, which on every
1679 C<eval()> is always set on failure and cleared on success.
1681 For more details, see the individual descriptions at C<$@>, C<$!>,
1686 =item ${^CHILD_ERROR_NATIVE}
1687 X<$^CHILD_ERROR_NATIVE>
1689 The native status returned by the last pipe close, backtick (C<``>)
1690 command, successful call to C<wait()> or C<waitpid()>, or from the
1691 C<system()> operator. On POSIX-like systems this value can be decoded
1692 with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1693 WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
1695 Under VMS this reflects the actual VMS exit status; i.e. it is the
1696 same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
1698 This variable was added in Perl v5.10.0.
1700 =item $EXTENDED_OS_ERROR
1703 X<$^E> X<$EXTENDED_OS_ERROR>
1705 Error information specific to the current operating system. At the
1706 moment, this differs from C<L</$!>> under only VMS, OS/2, and Win32 (and
1707 for MacPerl). On all other platforms, C<$^E> is always just the same
1710 Under VMS, C<$^E> provides the VMS status value from the last system
1711 error. This is more specific information about the last system error
1712 than that provided by C<$!>. This is particularly important when C<$!>
1713 is set to B<EVMSERR>.
1715 Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1716 API either via CRT, or directly from perl.
1718 Under Win32, C<$^E> always returns the last error information reported
1719 by the Win32 call C<GetLastError()> which describes the last error
1720 from within the Win32 API. Most Win32-specific code will report errors
1721 via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1722 portable Perl code will report errors via C<$!>.
1724 Caveats mentioned in the description of C<L</$!>> generally apply to
1727 This variable was added in Perl 5.003.
1729 Mnemonic: Extra error explanation.
1731 =item $EXCEPTIONS_BEING_CAUGHT
1734 X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1736 Current state of the interpreter.
1739 --------- -------------------------------------
1740 undef Parsing module, eval, or main program
1741 true (1) Executing an eval
1744 The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1747 The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because
1748 the C<undef> value does not indicate whether exceptions are being caught,
1749 since compilation of the main program does not catch exceptions.
1751 This variable was added in Perl 5.004.
1758 The current value of the warning switch, initially true if B<-w> was
1759 used, false otherwise, but directly modifiable.
1761 See also L<warnings>.
1763 Mnemonic: related to the B<-w> switch.
1765 =item ${^WARNING_BITS}
1768 The current set of warning checks enabled by the C<use warnings> pragma.
1769 It has the same scoping as the C<$^H> and C<%^H> variables. The exact
1770 values are considered internal to the L<warnings> pragma and may change
1771 between versions of Perl.
1773 This variable was added in Perl v5.6.0.
1780 X<$!> X<$ERRNO> X<$OS_ERROR>
1782 When referenced, C<$!> retrieves the current value
1783 of the C C<errno> integer variable.
1784 If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1785 When referenced as a string, C<$!> yields the system error string
1786 corresponding to C<errno>.
1788 Many system or library calls set C<errno> if they fail,
1789 to indicate the cause of failure. They usually do B<not>
1790 set C<errno> to zero if they succeed. This means C<errno>,
1791 hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1793 if (open my $fh, "<", $filename) {
1794 # Here $! is meaningless.
1798 # ONLY here is $! meaningful.
1800 # Already here $! might be meaningless.
1802 # Since here we might have either success or failure,
1803 # $! is meaningless.
1805 Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1806 of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1807 It can be used immediately before invoking the C<die()> operator,
1808 to set the exit value, or to inspect the system error string
1809 corresponding to error I<n>, or to restore C<$!> to a meaningful state.
1811 Mnemonic: What just went bang?
1818 X<%!> X<%OS_ERROR> X<%ERRNO>
1820 Each element of C<%!> has a true value only if C<$!> is set to that
1821 value. For example, C<$!{ENOENT}> is true if and only if the current
1822 value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1823 such file or directory" (or its moral equivalent: not all operating
1824 systems give that exact error, and certainly not all languages). The
1825 specific true value is not guaranteed, but in the past has generally
1826 been the numeric value of C<$!>. To check if a particular key is
1827 meaningful on your system, use C<exists $!{the_key}>; for a list of legal
1828 keys, use C<keys %!>. See L<Errno> for more information, and also see
1831 This variable was added in Perl 5.005.
1836 X<$?> X<$CHILD_ERROR>
1838 The status returned by the last pipe close, backtick (C<``>) command,
1839 successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1840 operator. This is just the 16-bit status word returned by the
1841 traditional Unix C<wait()> system call (or else is made up to look
1842 like it). Thus, the exit value of the subprocess is really (C<<< $? >>
1843 8 >>>), and C<$? & 127> gives which signal, if any, the process died
1844 from, and C<$? & 128> reports whether there was a core dump.
1846 Additionally, if the C<h_errno> variable is supported in C, its value
1847 is returned via C<$?> if any C<gethost*()> function fails.
1849 If you have installed a signal handler for C<SIGCHLD>, the
1850 value of C<$?> will usually be wrong outside that handler.
1852 Inside an C<END> subroutine C<$?> contains the value that is going to be
1853 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1854 change the exit status of your program. For example:
1857 $? = 1 if $? == 255; # die would make it 255
1860 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1861 actual VMS exit status, instead of the default emulation of POSIX
1862 status; see L<perlvms/$?> for details.
1864 Mnemonic: similar to B<sh> and B<ksh>.
1869 X<$@> X<$EVAL_ERROR>
1871 The Perl error from the last C<eval> operator, i.e. the last exception that
1872 was caught. For C<eval BLOCK>, this is either a runtime error message or the
1873 string or reference C<die> was called with. The C<eval STRING> form also
1874 catches syntax errors and other compile time exceptions.
1876 If no error occurs, C<eval> sets C<$@> to the empty string.
1878 Warning messages are not collected in this variable. You can, however,
1879 set up a routine to process warnings by setting C<$SIG{__WARN__}> as
1880 described in L</%SIG>.
1882 Mnemonic: Where was the error "at"?
1886 =head2 Variables related to the interpreter state
1888 These variables provide information about the current interpreter state.
1895 X<$^C> X<$COMPILING>
1897 The current value of the flag associated with the B<-c> switch.
1898 Mainly of use with B<-MO=...> to allow code to alter its behavior
1899 when being compiled, such as for example to C<AUTOLOAD> at compile
1900 time rather than normal, deferred loading. Setting
1901 C<$^C = 1> is similar to calling C<B::minus_c>.
1903 This variable was added in Perl v5.6.0.
1908 X<$^D> X<$DEBUGGING>
1910 The current value of the debugging flags. May be read or set. Like its
1911 L<command-line equivalent|perlrun/B<-D>I<letters>>, you can use numeric
1912 or symbolic values, e.g. C<$^D = 10> or C<$^D = "st">. See
1913 L<perlrun/B<-D>I<number>>. The contents of this variable also affects the
1914 debugger operation. See L<perldebguts/Debugger Internals>.
1916 Mnemonic: value of B<-D> switch.
1921 This variable is no longer supported.
1923 It used to hold the I<object reference> to the C<Encode> object that was
1924 used to convert the source code to Unicode.
1926 Its purpose was to allow your non-ASCII Perl
1927 scripts not to have to be written in UTF-8; this was
1928 useful before editors that worked on UTF-8 encoded text were common, but
1929 that was long ago. It caused problems, such as affecting the operation
1930 of other modules that weren't expecting it, causing general mayhem.
1932 If you need something like this functionality, it is recommended that use
1933 you a simple source filter, such as L<Filter::Encoding>.
1935 If you are coming here because code of yours is being adversely affected
1936 by someone's use of this variable, you can usually work around it by
1941 near the beginning of the functions that are getting broken. This
1942 undefines the variable during the scope of execution of the including
1945 This variable was added in Perl 5.8.2 and removed in 5.26.0.
1947 =item ${^GLOBAL_PHASE}
1950 The current phase of the perl interpreter.
1952 Possible values are:
1958 The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1959 value is mostly there for completeness and for use via the
1960 underlying C variable C<PL_phase>. It's not really possible for Perl
1961 code to be executed unless construction of the interpreter is
1966 This is the global compile-time. That includes, basically, every
1967 C<BEGIN> block executed directly or indirectly from during the
1968 compile-time of the top-level program.
1970 This phase is not called "BEGIN" to avoid confusion with
1971 C<BEGIN>-blocks, as those are executed during compile-time of any
1972 compilation unit, not just the top-level program. A new, localised
1973 compile-time entered at run-time, for example by constructs as
1974 C<eval "use SomeModule"> are not global interpreter phases, and
1975 therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1979 Execution of any C<CHECK> blocks.
1983 Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1987 The main run-time, i.e. the execution of C<PL_main_root>.
1991 Execution of any C<END> blocks.
1999 Also note that there's no value for UNITCHECK-blocks. That's because
2000 those are run for each compilation unit individually, and therefore is
2001 not a global interpreter phase.
2003 Not every program has to go through each of the possible phases, but
2004 transition from one phase to another can only happen in the order
2005 described in the above list.
2007 An example of all of the phases Perl code can see:
2009 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
2011 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
2013 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
2016 package Print::Phase;
2019 my ($class, $time) = @_;
2020 return bless \$time, $class;
2025 print "$$self: ${^GLOBAL_PHASE}\n";
2029 print "run-time: ${^GLOBAL_PHASE}\n";
2031 my $runtime = Print::Phase->new(
2032 "lexical variables are garbage collected before END"
2035 END { print "end-time: ${^GLOBAL_PHASE}\n" }
2037 our $destruct = Print::Phase->new(
2038 "package variables are garbage collected after END"
2047 lexical variables are garbage collected before END: RUN
2049 package variables are garbage collected after END: DESTRUCT
2051 This variable was added in Perl 5.14.0.
2056 WARNING: This variable is strictly for
2057 internal use only. Its availability,
2058 behavior, and contents are subject to change without notice.
2060 This variable contains compile-time hints for the Perl interpreter. At the
2061 end of compilation of a BLOCK the value of this variable is restored to the
2062 value when the interpreter started to compile the BLOCK.
2064 When perl begins to parse any block construct that provides a lexical scope
2065 (e.g., eval body, required file, subroutine body, loop body, or conditional
2066 block), the existing value of C<$^H> is saved, but its value is left unchanged.
2067 When the compilation of the block is completed, it regains the saved value.
2068 Between the points where its value is saved and restored, code that
2069 executes within BEGIN blocks is free to change the value of C<$^H>.
2071 This behavior provides the semantic of lexical scoping, and is used in,
2072 for instance, the C<use strict> pragma.
2074 The contents should be an integer; different bits of it are used for
2075 different pragmatic flags. Here's an example:
2077 sub add_100 { $^H |= 0x100 }
2084 Consider what happens during execution of the BEGIN block. At this point
2085 the BEGIN block has already been compiled, but the body of C<foo()> is still
2086 being compiled. The new value of C<$^H>
2087 will therefore be visible only while
2088 the body of C<foo()> is being compiled.
2090 Substitution of C<BEGIN { add_100() }> block with:
2092 BEGIN { require strict; strict->import('vars') }
2094 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
2095 version of the same lexical pragma:
2098 require strict; strict->import('vars') if $condition
2101 This variable was added in Perl 5.003.
2106 The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes
2107 it useful for implementation of lexically scoped pragmas. See
2108 L<perlpragma>. All the entries are stringified when accessed at
2109 runtime, so only simple values can be accommodated. This means no
2110 pointers to objects, for example.
2112 When putting items into C<%^H>, in order to avoid conflicting with other
2113 users of the hash there is a convention regarding which keys to use.
2114 A module should use only keys that begin with the module's name (the
2115 name of its main package) and a "/" character. For example, a module
2116 C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
2118 This variable was added in Perl v5.6.0.
2123 An internal variable used by PerlIO. A string in two parts, separated
2124 by a C<\0> byte, the first part describes the input layers, the second
2125 part describes the output layers.
2127 This variable was added in Perl v5.8.0.
2134 The internal variable for debugging support. The meanings of the
2135 various bits are subject to change, but currently indicate:
2141 Debug subroutine enter/exit.
2145 Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for
2146 each statement executed. Also causes saving source code lines (like
2151 Switch off optimizations.
2155 Preserve more data for future interactive inspections.
2159 Keep info about source lines on which a subroutine is defined.
2163 Start with single-step on.
2167 Use subroutine address instead of name when reporting.
2171 Report C<goto &subroutine> as well.
2175 Provide informative "file" names for evals based on the place they were compiled.
2179 Provide informative names to anonymous subroutines based on the place they
2184 Save source code lines into C<@{"_<$filename"}>.
2188 When saving source, include evals that generate no subroutines.
2192 When saving source, include source that did not compile.
2196 Some bits may be relevant at compile-time only, some at
2197 run-time only. This is a new mechanism and the details may change.
2198 See also L<perldebguts>.
2203 Reflects if taint mode is on or off. 1 for on (the program was run with
2204 B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2207 This variable is read-only.
2209 This variable was added in Perl v5.8.0.
2214 Reflects certain Unicode settings of Perl. See L<perlrun>
2215 documentation for the C<-C> switch for more information about
2216 the possible values.
2218 This variable is set during Perl startup and is thereafter read-only.
2220 This variable was added in Perl v5.8.2.
2225 This variable controls the state of the internal UTF-8 offset caching code.
2226 1 for on (the default), 0 for off, -1 to debug the caching code by checking
2227 all its results against linear scans, and panicking on any discrepancy.
2229 This variable was added in Perl v5.8.9. It is subject to change or
2230 removal without notice, but is currently used to avoid recalculating the
2231 boundaries of multi-byte UTF-8-encoded characters.
2233 =item ${^UTF8LOCALE}
2236 This variable indicates whether a UTF-8 locale was detected by perl at
2237 startup. This information is used by perl when it's in
2238 adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2239 switch); see L<perlrun> for more info on this.
2241 This variable was added in Perl v5.8.8.
2245 =head2 Deprecated and removed variables
2247 Deprecating a variable announces the intent of the perl maintainers to
2248 eventually remove the variable from the language. It may still be
2249 available despite its status. Using a deprecated variable triggers
2252 Once a variable is removed, its use triggers an error telling you
2253 the variable is unsupported.
2255 See L<perldiag> for details about error messages.
2262 C<$#> was a variable that could be used to format printed numbers.
2263 After a deprecation cycle, its magic was removed in Perl v5.10.0 and
2264 using it now triggers a warning: C<$# is no longer supported>.
2266 This is not the sigil you use in front of an array name to get the
2267 last index, like C<$#array>. That's still how you get the last index
2268 of an array in Perl. The two have nothing to do with each other.
2270 Deprecated in Perl 5.
2272 Removed in Perl v5.10.0.
2277 C<$*> was a variable that you could use to enable multiline matching.
2278 After a deprecation cycle, its magic was removed in Perl v5.10.0.
2279 Using it now triggers a warning: C<$* is no longer supported>.
2280 You should use the C</s> and C</m> regexp modifiers instead.
2282 Deprecated in Perl 5.
2284 Removed in Perl v5.10.0.
2289 This variable stores the index of the first element in an array, and
2290 of the first character in a substring. The default is 0, but you could
2291 theoretically set it to 1 to make Perl behave more like B<awk> (or Fortran)
2292 when subscripting and when evaluating the index() and substr() functions.
2294 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
2295 directive, and cannot influence the behavior of any other file.
2296 (That's why you can only assign compile-time constants to it.)
2297 Its use is highly discouraged.
2299 Prior to Perl v5.10.0, assignment to C<$[> could be seen from outer lexical
2300 scopes in the same file, unlike other compile-time directives (such as
2301 L<strict>). Using local() on it would bind its value strictly to a lexical
2302 block. Now it is always lexically scoped.
2304 As of Perl v5.16.0, it is implemented by the L<arybase> module. See
2305 L<arybase> for more details on its behaviour.
2307 Under C<use v5.16>, or C<no feature "array_base">, C<$[> no longer has any
2308 effect, and always contains 0. Assigning 0 to it is permitted, but any
2309 other value will produce an error.
2311 Mnemonic: [ begins subscripts.
2313 Deprecated in Perl v5.12.0.