This is a live mirror of the Perl 5 development currently hosted at https://github.com/perl/perl5
perlrecharclass: Nit
[perl5.git] / pod / perlvar.pod
CommitLineData
a0d0e21e
LW
1=head1 NAME
2
3perlvar - Perl predefined variables
4
5=head1 DESCRIPTION
6
b0c22438 7=head2 The Syntax of Variable Names
8
0b9346e6 9Variable names in Perl can have several formats. Usually, they
b0c22438 10must begin with a letter or underscore, in which case they can be
11arbitrarily long (up to an internal limit of 251 characters) and
12may contain letters, digits, underscores, or the special sequence
0b9346e6 13C<::> or C<'>. In this case, the part before the last C<::> or
b0c22438 14C<'> is taken to be a I<package qualifier>; see L<perlmod>.
15
16Perl variable names may also be a sequence of digits or a single
0b9346e6 17punctuation or control character. These names are all reserved for
b0c22438 18special uses by Perl; for example, the all-digits names are used
19to hold data captured by backreferences after a regular expression
0b9346e6 20match. Perl has a special syntax for the single-control-character
b0c22438 21names: It understands C<^X> (caret C<X>) to mean the control-C<X>
0b9346e6 22character. For example, the notation C<$^W> (dollar-sign caret
b0c22438 23C<W>) is the scalar variable whose name is the single character
0b9346e6 24control-C<W>. This is better than typing a literal control-C<W>
b0c22438 25into your program.
26
27Since Perl 5.6, Perl variable names may be alphanumeric
28strings that begin with control characters (or better yet, a caret).
29These variables must be written in the form C<${^Foo}>; the braces
0b9346e6 30are not optional. C<${^Foo}> denotes the scalar variable whose
31name is a control-C<F> followed by two C<o>'s. These variables are
b0c22438 32reserved for future special uses by Perl, except for the ones that
0b9346e6 33begin with C<^_> (control-underscore or caret-underscore). No
b0c22438 34control-character name that begins with C<^_> will acquire a special
35meaning in any future version of Perl; such names may therefore be
0b9346e6 36used safely in programs. C<$^_> itself, however, I<is> reserved.
b0c22438 37
38Perl identifiers that begin with digits, control characters, or
39punctuation characters are exempt from the effects of the C<package>
40declaration and are always forced to be in package C<main>; they are
0b9346e6 41also exempt from C<strict 'vars'> errors. A few other names are also
b0c22438 42exempt in these ways:
43
0b9346e6 44 ENV STDIN
45 INC STDOUT
46 ARGV STDERR
47 ARGVOUT
b0c22438 48 SIG
49
69520822 50In particular, the special C<${^_XYZ}> variables are always taken
b0c22438 51to be in package C<main>, regardless of any C<package> declarations
52presently in scope.
53
54=head1 SPECIAL VARIABLES
a0d0e21e 55
0b9346e6 56The following names have special meaning to Perl. Most punctuation
57names have reasonable mnemonics, or analogs in the shells.
58Nevertheless, if you wish to use long variable names, you need only say:
a0d0e21e 59
0b9346e6 60 use English;
a0d0e21e 61
a1ce9542
JF
62at the top of your program. This aliases all the short names to the long
63names in the current package. Some even have medium names, generally
84dabc03 64borrowed from B<awk>. To avoid a performance hit, if you don't need the
65C<$PREMATCH>, C<$MATCH>, or C<$POSTMATCH> it's best to use the C<English>
66module without them:
a0d0e21e 67
0b9346e6 68 use English '-no_match_vars';
a1ce9542 69
0b9346e6 70Before you continue, note the sort order for variables. In general, we
71first list the variables in case-insensitive, almost-lexigraphical
72order (ignoring the C<{> or C<^> preceding words, as in C<${^UNICODE}>
73or C<$^T>), although C<$_> and C<@_> move up to the top of the pile.
74For variables with the same identifier, we list it in order of scalar,
75array, hash, and bareword.
a1ce9542 76
b0c22438 77=head2 General Variables
a0d0e21e 78
84dabc03 79=over 8
80
a0d0e21e
LW
81=item $ARG
82
83=item $_
a054c801 84X<$_> X<$ARG>
a0d0e21e 85
b0c22438 86The default input and pattern-searching space. The following pairs are
a0d0e21e
LW
87equivalent:
88
0b9346e6 89 while (<>) {...} # equivalent only in while!
90 while (defined($_ = <>)) {...}
a0d0e21e 91
0b9346e6 92 /^Subject:/
93 $_ =~ /^Subject:/
a0d0e21e 94
0b9346e6 95 tr/a-z/A-Z/
96 $_ =~ tr/a-z/A-Z/
a0d0e21e 97
0b9346e6 98 chomp
99 chomp($_)
a0d0e21e 100
0b9346e6 101Here are the places where Perl will assume C<$_> even if you don't use it:
cb1a09d0
AD
102
103=over 3
104
105=item *
106
84dabc03 107The following functions use C<$_> as a default argument:
db1511c8 108
b0169937
GS
109abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob,
110hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print,
111quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only),
b0c18621 112rmdir, sin, split (on its second argument), sqrt, stat, study, uc, ucfirst,
b0169937 113unlink, unpack.
cb1a09d0
AD
114
115=item *
116
db1511c8
GS
117All file tests (C<-f>, C<-d>) except for C<-t>, which defaults to STDIN.
118See L<perlfunc/-X>
119
cb1a09d0
AD
120=item *
121
b0169937
GS
122The pattern matching operations C<m//>, C<s///> and C<tr///> (aka C<y///>)
123when used without an C<=~> operator.
cb1a09d0 124
54310121 125=item *
cb1a09d0
AD
126
127The default iterator variable in a C<foreach> loop if no other
128variable is supplied.
129
54310121 130=item *
cb1a09d0 131
b0c22438 132The implicit iterator variable in the C<grep()> and C<map()> functions.
cb1a09d0 133
54310121 134=item *
cb1a09d0 135
b0c22438 136The implicit variable of C<given()>.
db1511c8
GS
137
138=item *
139
c47ff5f1 140The default place to put an input record when a C<< <FH> >>
cb1a09d0 141operation's result is tested by itself as the sole criterion of a C<while>
b0c22438 142test. Outside a C<while> test, this will not happen.
cb1a09d0
AD
143
144=back
145
59f00321 146As C<$_> is a global variable, this may lead in some cases to unwanted
b0c22438 147side-effects. As of perl 5.9.1, you can now use a lexical version of
148C<$_> by declaring it in a file or in a block with C<my>. Moreover,
4fd88bf8 149declaring C<our $_> restores the global C<$_> in the current scope.
59f00321 150
b0c22438 151Mnemonic: underline is understood in certain operations.
a0d0e21e 152
0b9346e6 153=item @ARG
cde0cee5 154
0b9346e6 155=item @_
156X<@_> X<@ARG>
a0d0e21e 157
0b9346e6 158Within a subroutine the array C<@_> contains the parameters passed to
159that subroutine. Inside a subroutine, C<@_> is the default array for
160the array operators C<push>, C<pop>, C<shift>, and C<unshift>.
a0d0e21e 161
0b9346e6 162See L<perlsub>.
a0d0e21e 163
1311257d 164=item $LIST_SEPARATOR
165
166=item $"
167X<$"> X<$LIST_SEPARATOR>
168
69520822 169When an array or an array slice is interpolated into a double-quoted
170string or a similar context such as C</.../>, its elements are
171separated by this value. Default is a space. For example, this:
172
0b9346e6 173 print "The array is: @array\n";
69520822 174
175is equivalent to this:
176
0b9346e6 177 print "The array is: " . join($", @array) . "\n";
69520822 178
179Mnemonic: works in double-quoted context.
1311257d 180
b0c22438 181=item $PROCESS_ID
cde0cee5 182
b0c22438 183=item $PID
a0d0e21e 184
b0c22438 185=item $$
186X<$$> X<$PID> X<$PROCESS_ID>
a0d0e21e 187
4a904372
FC
188The process number of the Perl running this script. Though you I<can> set
189this variable, doing so is generally discouraged, although it can be
190invaluable for some testing purposes. It will be reset automatically
b0c22438 191across C<fork()> calls.
a0d0e21e 192
b0c22438 193Note for Linux users: on Linux, the C functions C<getpid()> and
194C<getppid()> return different values from different threads. In order to
195be portable, this behavior is not reflected by C<$$>, whose value remains
196consistent across threads. If you want to call the underlying C<getpid()>,
197you may use the CPAN module C<Linux::Pid>.
a0d0e21e 198
b0c22438 199Mnemonic: same as shells.
ad83b128 200
66d7055b
DR
201=item $PROGRAM_NAME
202
203=item $0
204X<$0> X<$PROGRAM_NAME>
205
206Contains the name of the program being executed.
207
208On some (but not all) operating systems assigning to C<$0> modifies
209the argument area that the C<ps> program sees. On some platforms you
210may have to use special C<ps> options or a different C<ps> to see the
211changes. Modifying the C<$0> is more useful as a way of indicating the
212current program state than it is for hiding the program you're
213running.
214
215Note that there are platform-specific limitations on the maximum
216length of C<$0>. In the most extreme case it may be limited to the
217space occupied by the original C<$0>.
218
219In some platforms there may be arbitrary amount of padding, for
220example space characters, after the modified name as shown by C<ps>.
221In some platforms this padding may extend all the way to the original
222length of the argument area, no matter what you do (this is the case
223for example with Linux 2.2).
224
225Note for BSD users: setting C<$0> does not completely remove "perl"
226from the ps(1) output. For example, setting C<$0> to C<"foobar"> may
227result in C<"perl: foobar (perl)"> (whether both the C<"perl: "> prefix
228and the " (perl)" suffix are shown depends on your exact BSD variant
229and version). This is an operating system feature, Perl cannot help it.
230
231In multithreaded scripts Perl coordinates the threads so that any
232thread may modify its copy of the C<$0> and the change becomes visible
233to ps(1) (assuming the operating system plays along). Note that
234the view of C<$0> the other threads have will not change since they
235have their own copies of it.
236
237If the program has been given to perl via the switches C<-e> or C<-E>,
238C<$0> will contain the string C<"-e">.
239
240On Linux as of perl 5.14 the legacy process name will be set with
241C<prctl(2)>, in addition to altering the POSIX name via C<argv[0]> as
242perl has done since version 4.000. Now system utilities that read the
243legacy process name such as ps, top and killall will recognize the
244name you set when assigning to C<$0>. The string you supply will be
245cut off at 16 bytes, this is a limitation imposed by Linux.
246
247Mnemonic: same as B<sh> and B<ksh>.
248
b0c22438 249=item $REAL_GROUP_ID
a01268b5 250
b0c22438 251=item $GID
a01268b5 252
b0c22438 253=item $(
254X<$(> X<$GID> X<$REAL_GROUP_ID>
a01268b5 255
b0c22438 256The real gid of this process. If you are on a machine that supports
257membership in multiple groups simultaneously, gives a space separated
258list of groups you are in. The first number is the one returned by
259C<getgid()>, and the subsequent ones by C<getgroups()>, one of which may be
260the same as the first number.
a01268b5 261
b0c22438 262However, a value assigned to C<$(> must be a single number used to
263set the real gid. So the value given by C<$(> should I<not> be assigned
264back to C<$(> without being forced numeric, such as by adding zero. Note
265that this is different to the effective gid (C<$)>) which does take a
266list.
fe307981 267
b0c22438 268You can change both the real gid and the effective gid at the same
269time by using C<POSIX::setgid()>. Changes to C<$(> require a check to C<$!>
270to detect any possible errors after an attempted change.
6cef1e77 271
b0c22438 272Mnemonic: parentheses are used to I<group> things. The real gid is the
273group you I<left>, if you're running setgid.
6cef1e77 274
b0c22438 275=item $EFFECTIVE_GROUP_ID
8e08999f 276
b0c22438 277=item $EGID
81714fb9 278
b0c22438 279=item $)
280X<$)> X<$EGID> X<$EFFECTIVE_GROUP_ID>
81714fb9 281
b0c22438 282The effective gid of this process. If you are on a machine that
283supports membership in multiple groups simultaneously, gives a space
284separated list of groups you are in. The first number is the one
285returned by C<getegid()>, and the subsequent ones by C<getgroups()>,
286one of which may be the same as the first number.
81714fb9 287
b0c22438 288Similarly, a value assigned to C<$)> must also be a space-separated
289list of numbers. The first number sets the effective gid, and
290the rest (if any) are passed to C<setgroups()>. To get the effect of an
291empty list for C<setgroups()>, just repeat the new effective gid; that is,
292to force an effective gid of 5 and an effectively empty C<setgroups()>
293list, say C< $) = "5 5" >.
81714fb9 294
b0c22438 295You can change both the effective gid and the real gid at the same
296time by using C<POSIX::setgid()> (use only a single numeric argument).
297Changes to C<$)> require a check to C<$!> to detect any possible errors
298after an attempted change.
44a2ac75 299
b0c22438 300C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
301machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
302and C<$)> can be swapped only on machines supporting C<setregid()>.
3195cf34 303
b0c22438 304Mnemonic: parentheses are used to I<group> things. The effective gid
305is the group that's I<right> for you, if you're running setgid.
44a2ac75 306
c82f2f4e
DR
307=item $REAL_USER_ID
308
309=item $UID
310
311=item $<
312X<< $< >> X<$UID> X<$REAL_USER_ID>
313
314The real uid of this process. You can change both the real uid and the
315effective uid at the same time by using C<POSIX::setuid()>. Since
316changes to C<< $< >> require a system call, check C<$!> after a change
317attempt to detect any possible errors.
318
319Mnemonic: it's the uid you came I<from>, if you're running setuid.
320
321=item $EFFECTIVE_USER_ID
322
323=item $EUID
324
325=item $>
326X<< $> >> X<$EUID> X<$EFFECTIVE_USER_ID>
327
328The effective uid of this process. For example:
329
330 $< = $>; # set real to effective uid
331 ($<,$>) = ($>,$<); # swap real and effective uids
332
333You can change both the effective uid and the real uid at the same
334time by using C<POSIX::setuid()>. Changes to C<< $> >> require a check
335to C<$!> to detect any possible errors after an attempted change.
336
337C<< $< >> and C<< $> >> can be swapped only on machines
338supporting C<setreuid()>.
339
340Mnemonic: it's the uid you went I<to>, if you're running setuid.
341
0b9346e6 342=item $SUBSCRIPT_SEPARATOR
343
344=item $SUBSEP
345
346=item $;
347X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>
348
349The subscript separator for multidimensional array emulation. If you
350refer to a hash element as
351
352 $foo{$a,$b,$c}
353
354it really means
355
356 $foo{join($;, $a, $b, $c)}
357
358But don't put
359
360 @foo{$a,$b,$c} # a slice--note the @
361
362which means
363
364 ($foo{$a},$foo{$b},$foo{$c})
365
7333b1c4 366Default is "\034", the same as SUBSEP in B<awk>. If your keys contain
0b9346e6 367binary data there might not be any safe value for C<$;>.
368
369Consider using "real" multidimensional arrays as described
370in L<perllol>.
371
372Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.
373
0b9346e6 374=item $a
375
376=item $b
377X<$a> X<$b>
378
379Special package variables when using C<sort()>, see L<perlfunc/sort>.
380Because of this specialness C<$a> and C<$b> don't need to be declared
381(using C<use vars>, or C<our()>) even when using the C<strict 'vars'>
382pragma. Don't lexicalize them with C<my $a> or C<my $b> if you want to
383be able to use them in the C<sort()> comparison block or function.
384
0b9346e6 385=item %ENV
386X<%ENV>
387
388The hash C<%ENV> contains your current environment. Setting a
389value in C<ENV> changes the environment for any child processes
390you subsequently C<fork()> off.
391
b0c22438 392=item $SYSTEM_FD_MAX
5b2b9c68 393
b0c22438 394=item $^F
395X<$^F> X<$SYSTEM_FD_MAX>
5b2b9c68 396
b0c22438 397The maximum system file descriptor, ordinarily 2. System file
398descriptors are passed to C<exec()>ed processes, while higher file
399descriptors are not. Also, during an C<open()>, system file descriptors are
400preserved even if the C<open()> fails (ordinary file descriptors are
401closed before the C<open()> is attempted). The close-on-exec
402status of a file descriptor will be decided according to the value of
403C<$^F> when the corresponding file, pipe, or socket was opened, not the
404time of the C<exec()>.
5b2b9c68 405
0b9346e6 406=item @F
407X<@F>
408
409The array C<@F> contains the fields of each line read in when autosplit
7333b1c4 410mode is turned on. See L<perlrun> for the B<-a> switch. This array
0b9346e6 411is package-specific, and must be declared or given a full package name
412if not in package main when running under C<strict 'vars'>.
413
0b9346e6 414=item @INC
415X<@INC>
416
417The array C<@INC> contains the list of places that the C<do EXPR>,
7333b1c4 418C<require>, or C<use> constructs look for their library files. It
0b9346e6 419initially consists of the arguments to any B<-I> command-line
420switches, followed by the default Perl library, probably
421F</usr/local/lib/perl>, followed by ".", to represent the current
7333b1c4 422directory. ("." will not be appended if taint checks are enabled,
0b9346e6 423either by C<-T> or by C<-t>.) If you need to modify this at runtime,
424you should use the C<use lib> pragma to get the machine-dependent
425library properly loaded also:
426
427 use lib '/mypath/libdir/';
428 use SomeMod;
429
430You can also insert hooks into the file inclusion system by putting Perl
431code directly into C<@INC>. Those hooks may be subroutine references, array
432references or blessed objects. See L<perlfunc/require> for details.
433
434=item %INC
435X<%INC>
436
437The hash C<%INC> contains entries for each filename included via the
438C<do>, C<require>, or C<use> operators. The key is the filename
439you specified (with module names converted to pathnames), and the
440value is the location of the file found. The C<require>
441operator uses this hash to determine whether a particular file has
442already been included.
443
444If the file was loaded via a hook (e.g. a subroutine reference, see
445L<perlfunc/require> for a description of these hooks), this hook is
446by default inserted into C<%INC> in place of a filename. Note, however,
447that the hook may have set the C<%INC> entry by itself to provide some more
448specific info.
449
b0c22438 450=item $INPLACE_EDIT
a0d0e21e 451
b0c22438 452=item $^I
453X<$^I> X<$INPLACE_EDIT>
a0d0e21e 454
b0c22438 455The current value of the inplace-edit extension. Use C<undef> to disable
456inplace editing.
a0d0e21e 457
b0c22438 458Mnemonic: value of B<-i> switch.
a0d0e21e 459
b0c22438 460=item $^M
461X<$^M>
a0d0e21e 462
b0c22438 463By default, running out of memory is an untrappable, fatal error.
464However, if suitably built, Perl can use the contents of C<$^M>
465as an emergency memory pool after C<die()>ing. Suppose that your Perl
466were compiled with C<-DPERL_EMERGENCY_SBRK> and used Perl's malloc.
467Then
a0d0e21e 468
0b9346e6 469 $^M = 'a' x (1 << 16);
a0d0e21e 470
b0c22438 471would allocate a 64K buffer for use in an emergency. See the
472F<INSTALL> file in the Perl distribution for information on how to
473add custom C compilation flags when compiling perl. To discourage casual
474use of this advanced feature, there is no L<English|English> long name for
475this variable.
a0d0e21e 476
b0c22438 477This variable was added in Perl 5.004.
a0d0e21e 478
b0c22438 479=item $OSNAME
a0d0e21e 480
b0c22438 481=item $^O
482X<$^O> X<$OSNAME>
a0d0e21e 483
b0c22438 484The name of the operating system under which this copy of Perl was
485built, as determined during the configuration process. For examples
486see L<perlport/PLATFORMS>.
a0d0e21e 487
b0c22438 488The value is identical to C<$Config{'osname'}>. See also L<Config>
489and the B<-V> command-line switch documented in L<perlrun>.
a0d0e21e 490
b0c22438 491In Windows platforms, C<$^O> is not very helpful: since it is always
492C<MSWin32>, it doesn't tell the difference between
49395/98/ME/NT/2000/XP/CE/.NET. Use C<Win32::GetOSName()> or
494Win32::GetOSVersion() (see L<Win32> and L<perlport>) to distinguish
495between the variants.
a0d0e21e 496
b0c22438 497This variable was added in Perl 5.003.
a0d0e21e 498
1fa81471
DR
499=item %SIG
500X<%SIG>
a0d0e21e 501
1fa81471 502The hash C<%SIG> contains signal handlers for signals. For example:
a0d0e21e 503
1fa81471
DR
504 sub handler { # 1st argument is signal name
505 my($sig) = @_;
506 print "Caught a SIG$sig--shutting down\n";
507 close(LOG);
508 exit(0);
509 }
a0d0e21e 510
1fa81471
DR
511 $SIG{'INT'} = \&handler;
512 $SIG{'QUIT'} = \&handler;
513 ...
514 $SIG{'INT'} = 'DEFAULT'; # restore default action
515 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
a0d0e21e 516
1fa81471
DR
517Using a value of C<'IGNORE'> usually has the effect of ignoring the
518signal, except for the C<CHLD> signal. See L<perlipc> for more about
519this special case.
a0d0e21e 520
1fa81471 521Here are some other examples:
a0d0e21e 522
1fa81471
DR
523 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
524 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
525 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
526 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
a0d0e21e 527
1fa81471
DR
528Be sure not to use a bareword as the name of a signal handler,
529lest you inadvertently call it.
a0d0e21e 530
1fa81471
DR
531If your system has the C<sigaction()> function then signal handlers
532are installed using it. This means you get reliable signal handling.
a0d0e21e 533
1fa81471
DR
534The default delivery policy of signals changed in Perl 5.8.0 from
535immediate (also known as "unsafe") to deferred, also known as "safe
536signals". See L<perlipc> for more information.
a0d0e21e 537
1fa81471
DR
538Certain internal hooks can be also set using the C<%SIG> hash. The
539routine indicated by C<$SIG{__WARN__}> is called when a warning
540message is about to be printed. The warning message is passed as the
541first argument. The presence of a C<__WARN__> hook causes the
542ordinary printing of warnings to C<STDERR> to be suppressed. You can
543use this to save warnings in a variable, or turn warnings into fatal
544errors, like this:
a0d0e21e 545
1fa81471
DR
546 local $SIG{__WARN__} = sub { die $_[0] };
547 eval $proggie;
a8f8344d 548
b0c22438 549As the C<'IGNORE'> hook is not supported by C<__WARN__>, you can
550disable warnings using the empty subroutine:
f86702cc 551
0b9346e6 552 local $SIG{__WARN__} = sub {};
55602bd2 553
b0c22438 554The routine indicated by C<$SIG{__DIE__}> is called when a fatal
555exception is about to be thrown. The error message is passed as the
556first argument. When a C<__DIE__> hook routine returns, the exception
557processing continues as it would have in the absence of the hook,
558unless the hook routine itself exits via a C<goto>, a loop exit, or a
559C<die()>. The C<__DIE__> handler is explicitly disabled during the
560call, so that you can die from a C<__DIE__> handler. Similarly for
561C<__WARN__>.
e5218da5 562
b0c22438 563Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
564even inside an C<eval()>. Do not use this to rewrite a pending
565exception in C<$@>, or as a bizarre substitute for overriding
566C<CORE::GLOBAL::die()>. This strange action at a distance may be fixed
567in a future release so that C<$SIG{__DIE__}> is only called if your
568program is about to exit, as was the original intent. Any other use is
569deprecated.
570
571C<__DIE__>/C<__WARN__> handlers are very special in one respect: they
572may be called to report (probable) errors found by the parser. In such
573a case the parser may be in inconsistent state, so any attempt to
574evaluate Perl code from such a handler will probably result in a
575segfault. This means that warnings or errors that result from parsing
576Perl should be used with extreme caution, like this:
e5218da5 577
0b9346e6 578 require Carp if defined $^S;
579 Carp::confess("Something wrong") if defined &Carp::confess;
580 die "Something wrong, but could not load Carp to give backtrace...
581 To see backtrace try starting Perl with -MCarp switch";
e5218da5 582
b0c22438 583Here the first line will load C<Carp> I<unless> it is the parser who
584called the handler. The second line will print backtrace and die if
585C<Carp> was available. The third line will be executed only if C<Carp> was
586not available.
0a378802 587
0b9346e6 588Having to even think about the C<$^S> variable in your exception
7333b1c4 589handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
0b9346e6 590invites grievous and difficult to track down errors. Avoid it
591and use an C<END{}> or CORE::GLOBAL::die override instead.
592
b0c22438 593See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
594L<warnings> for additional information.
0a378802 595
b0c22438 596=item $BASETIME
6ab308ee 597
b0c22438 598=item $^T
599X<$^T> X<$BASETIME>
6ab308ee 600
b0c22438 601The time at which the program began running, in seconds since the
602epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
603and B<-C> filetests are based on this value.
a0d0e21e 604
b0c22438 605=item $PERL_VERSION
a0d0e21e 606
b0c22438 607=item $^V
608X<$^V> X<$PERL_VERSION>
a0d0e21e 609
b0c22438 610The revision, version, and subversion of the Perl interpreter,
611represented as a C<version> object.
748a9306 612
b0c22438 613This variable first appeared in perl 5.6.0; earlier versions of perl
614will see an undefined value. Before perl 5.10.0 C<$^V> was represented
615as a v-string.
55602bd2 616
b0c22438 617C<$^V> can be used to determine whether the Perl interpreter executing
618a script is in the right range of versions. For example:
a0d0e21e 619
0b9346e6 620 warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
a0d0e21e 621
b0c22438 622To convert C<$^V> into its string representation use C<sprintf()>'s
623C<"%vd"> conversion:
a0d0e21e 624
0b9346e6 625 printf "version is v%vd\n", $^V; # Perl's version
a0d0e21e 626
b0c22438 627See the documentation of C<use VERSION> and C<require VERSION>
628for a convenient way to fail if the running Perl interpreter is too old.
4d76a344 629
b0c22438 630See also C<$]> for an older representation of the Perl version.
a0d0e21e 631
b0c22438 632This variable was added in Perl 5.6.
a0d0e21e 633
b0c22438 634Mnemonic: use ^V for Version Control.
a0d0e21e 635
b0c22438 636=item ${^WIN32_SLOPPY_STAT}
5b442a2a 637X<${^WIN32_SLOPPY_STAT}> X<sitecustomize> X<sitecustomize.pl>
a0d0e21e 638
b0c22438 639If this variable is set to a true value, then C<stat()> on Windows will
640not try to open the file. This means that the link count cannot be
641determined and file attributes may be out of date if additional
642hardlinks to the file exist. On the other hand, not opening the file
643is considerably faster, especially for files on network drives.
a0d0e21e 644
b0c22438 645This variable could be set in the F<sitecustomize.pl> file to
646configure the local Perl installation to use "sloppy" C<stat()> by
647default. See the documentation for B<-f> in
648L<perlrun|perlrun/"Command Switches"> for more information about site
649customization.
a0d0e21e 650
b0c22438 651This variable was added in Perl 5.10.
a0d0e21e 652
b0c22438 653=item $EXECUTABLE_NAME
a0d0e21e 654
b0c22438 655=item $^X
656X<$^X> X<$EXECUTABLE_NAME>
a0d0e21e 657
b0c22438 658The name used to execute the current copy of Perl, from C's
659C<argv[0]> or (where supported) F</proc/self/exe>.
a043a685 660
b0c22438 661Depending on the host operating system, the value of C<$^X> may be
662a relative or absolute pathname of the perl program file, or may
663be the string used to invoke perl but not the pathname of the
664perl program file. Also, most operating systems permit invoking
665programs that are not in the PATH environment variable, so there
666is no guarantee that the value of C<$^X> is in PATH. For VMS, the
667value may or may not include a version number.
a0d0e21e 668
b0c22438 669You usually can use the value of C<$^X> to re-invoke an independent
670copy of the same perl that is currently running, e.g.,
a0d0e21e 671
0b9346e6 672 @first_run = `$^X -le "print int rand 100 for 1..100"`;
a0d0e21e 673
b0c22438 674But recall that not all operating systems support forking or
675capturing of the output of commands, so this complex statement
676may not be portable.
a0d0e21e 677
b0c22438 678It is not safe to use the value of C<$^X> as a path name of a file,
679as some operating systems that have a mandatory suffix on
680executable files do not require use of the suffix when invoking
681a command. To convert the value of C<$^X> to a path name, use the
682following statements:
8cc95fdb 683
0b9346e6 684 # Build up a set of file names (not command names).
685 use Config;
686 my $this_perl = $^X;
687 if ($^O ne 'VMS') {
688 $this_perl .= $Config{_exe}
689 unless $this_perl =~ m/$Config{_exe}$/i;
690 }
8cc95fdb 691
b0c22438 692Because many operating systems permit anyone with read access to
693the Perl program file to make a copy of it, patch the copy, and
694then execute the copy, the security-conscious Perl programmer
695should take care to invoke the installed copy of perl, not the
696copy referenced by C<$^X>. The following statements accomplish
697this goal, and produce a pathname that can be invoked as a
698command or referenced as a file.
a043a685 699
0b9346e6 700 use Config;
701 my $secure_perl_path = $Config{perlpath};
702 if ($^O ne 'VMS') {
703 $secure_perl_path .= $Config{_exe}
704 unless $secure_perl_path =~ m/$Config{_exe}$/i;
705 }
a0d0e21e 706
b0c22438 707=back
a0d0e21e 708
b0c22438 709=head2 Variables related to regular expressions
710
711Most of the special variables related to regular expressions are side
712effects. Perl sets these variables when it has a successful match, so
713you should check the match result before using them. For instance:
714
715 if( /P(A)TT(ER)N/ ) {
716 print "I found $1 and $2\n";
717 }
718
0b9346e6 719These variables are read-only and dynamically-scoped, unless we note
b0c22438 720otherwise.
721
0b9346e6 722The dynamic nature of the regular expression variables means that
723their value is limited to the block that they are in, as demonstrated
724by this bit of code:
b0c22438 725
726 my $outer = 'Wallace and Grommit';
727 my $inner = 'Mutt and Jeff';
0b9346e6 728
b0c22438 729 my $pattern = qr/(\S+) and (\S+)/;
0b9346e6 730
b0c22438 731 sub show_n { print "\$1 is $1; \$2 is $2\n" }
0b9346e6 732
b0c22438 733 {
734 OUTER:
735 show_n() if $outer =~ m/$pattern/;
0b9346e6 736
b0c22438 737 INNER: {
738 show_n() if $inner =~ m/$pattern/;
739 }
0b9346e6 740
b0c22438 741 show_n();
742 }
743
0b9346e6 744The output shows that while in the C<OUTER> block, the values of C<$1>
745and C<$2> are from the match against C<$outer>. Inside the C<INNER>
746block, the values of C<$1> and C<$2> are from the match against
747C<$inner>, but only until the end of the block (i.e. the dynamic
748scope). After the C<INNER> block completes, the values of C<$1> and
749C<$2> return to the values for the match against C<$outer> even though
b0c22438 750we have not made another match:
751
752 $1 is Wallace; $2 is Grommit
753 $1 is Mutt; $2 is Jeff
754 $1 is Wallace; $2 is Grommit
a0d0e21e 755
0b9346e6 756Due to an unfortunate accident of Perl's implementation, C<use
757English> imposes a considerable performance penalty on all regular
758expression matches in a program because it uses the C<$`>, C<$&>, and
759C<$'>, regardless of whether they occur in the scope of C<use
760English>. For that reason, saying C<use English> in libraries is
761strongly discouraged unless you import it without the match variables:
762
763 use English '-no_match_vars'
764
d8a75b5a
FC
765The C<Devel::NYTProf> and C<Devel::FindAmpersand>
766modules can help you find uses of these
0b9346e6 767problematic match variables in your code.
768
769Since Perl 5.10, you can use the C</p> match operator flag and the
770C<${^PREMATCH}>, C<${^MATCH}>, and C<${^POSTMATCH}> variables instead
771so you only suffer the performance penalties.
772
b0c22438 773=over 8
a0d0e21e 774
b0c22438 775=item $<I<digits>> ($1, $2, ...)
776X<$1> X<$2> X<$3>
8cc95fdb 777
b0c22438 778Contains the subpattern from the corresponding set of capturing
779parentheses from the last successful pattern match, not counting patterns
780matched in nested blocks that have been exited already.
8cc95fdb 781
b0c22438 782These variables are read-only and dynamically-scoped.
a043a685 783
b0c22438 784Mnemonic: like \digits.
a0d0e21e 785
b0c22438 786=item $MATCH
a0d0e21e 787
b0c22438 788=item $&
789X<$&> X<$MATCH>
a0d0e21e 790
b0c22438 791The string matched by the last successful pattern match (not counting
792any matches hidden within a BLOCK or C<eval()> enclosed by the current
793BLOCK).
a0d0e21e 794
b0c22438 795The use of this variable anywhere in a program imposes a considerable
0b9346e6 796performance penalty on all regular expression matches. To avoid this
797penalty, you can extract the same substring by using L</@->. Starting
450a7a1b 798with Perl 5.10, you can use the C</p> match flag and the C<${^MATCH}>
0b9346e6 799variable to do the same thing for particular match operations.
80bca1b4 800
b0c22438 801This variable is read-only and dynamically-scoped.
f9cbb277 802
b0c22438 803Mnemonic: like C<&> in some editors.
0b9346e6 804
b0c22438 805=item ${^MATCH}
806X<${^MATCH}>
a0d0e21e 807
b0c22438 808This is similar to C<$&> (C<$MATCH>) except that it does not incur the
809performance penalty associated with that variable, and is only guaranteed
810to return a defined value when the pattern was compiled or executed with
811the C</p> modifier.
80bca1b4 812
b0c22438 813This variable was added in Perl 5.10.
4bc88a62 814
b0c22438 815This variable is read-only and dynamically-scoped.
e2975953 816
b0c22438 817=item $PREMATCH
52c447a8 818
b0c22438 819=item $`
5b442a2a 820X<$`> X<$PREMATCH> X<${^PREMATCH}>
7636ea95 821
b0c22438 822The string preceding whatever was matched by the last successful
823pattern match, not counting any matches hidden within a BLOCK or C<eval>
0b9346e6 824enclosed by the current BLOCK.
a0d0e21e 825
b0c22438 826The use of this variable anywhere in a program imposes a considerable
0b9346e6 827performance penalty on all regular expression matches. To avoid this
828penalty, you can extract the same substring by using L</@->. Starting
450a7a1b 829with Perl 5.10, you can use the C</p> match flag and the
0b9346e6 830C<${^PREMATCH}> variable to do the same thing for particular match
831operations.
a0d0e21e 832
b0c22438 833This variable is read-only and dynamically-scoped.
a0d0e21e 834
b0c22438 835Mnemonic: C<`> often precedes a quoted string.
f83ed198 836
b0c22438 837=item ${^PREMATCH}
5b442a2a 838X<$`> X<${^PREMATCH}>
a0d0e21e 839
b0c22438 840This is similar to C<$`> ($PREMATCH) except that it does not incur the
841performance penalty associated with that variable, and is only guaranteed
842to return a defined value when the pattern was compiled or executed with
843the C</p> modifier.
a0d0e21e 844
b0c22438 845This variable was added in Perl 5.10
a0d0e21e 846
b0c22438 847This variable is read-only and dynamically-scoped.
a0d0e21e 848
b0c22438 849=item $POSTMATCH
16070b82 850
b0c22438 851=item $'
5b442a2a 852X<$'> X<$POSTMATCH> X<${^POSTMATCH}> X<@->
305aace0 853
b0c22438 854The string following whatever was matched by the last successful
855pattern match (not counting any matches hidden within a BLOCK or C<eval()>
856enclosed by the current BLOCK). Example:
305aace0 857
0b9346e6 858 local $_ = 'abcdefghi';
859 /def/;
860 print "$`:$&:$'\n"; # prints abc:def:ghi
305aace0 861
b0c22438 862The use of this variable anywhere in a program imposes a considerable
0b9346e6 863performance penalty on all regular expression matches.
864To avoid this penalty, you can extract the same substring by
450a7a1b 865using L</@->. Starting with Perl 5.10, you can use the C</p> match flag
0b9346e6 866and the C<${^POSTMATCH}> variable to do the same thing for particular
b0c22438 867match operations.
a0d0e21e 868
b0c22438 869This variable is read-only and dynamically-scoped.
870
871Mnemonic: C<'> often follows a quoted string.
872
873=item ${^POSTMATCH}
5b442a2a 874X<${^POSTMATCH}> X<$'> X<$POSTMATCH>
b0c22438 875
876This is similar to C<$'> (C<$POSTMATCH>) except that it does not incur the
877performance penalty associated with that variable, and is only guaranteed
878to return a defined value when the pattern was compiled or executed with
879the C</p> modifier.
880
881This variable was added in Perl 5.10.
882
883This variable is read-only and dynamically-scoped.
884
885=item $LAST_PAREN_MATCH
886
887=item $+
888X<$+> X<$LAST_PAREN_MATCH>
889
890The text matched by the last bracket of the last successful search pattern.
891This is useful if you don't know which one of a set of alternative patterns
892matched. For example:
893
0b9346e6 894 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
b0c22438 895
896This variable is read-only and dynamically-scoped.
897
898Mnemonic: be positive and forward looking.
899
900=item $LAST_SUBMATCH_RESULT
901
902=item $^N
5b442a2a 903X<$^N> X<$LAST_SUBMATCH_RESULT>
b0c22438 904
905The text matched by the used group most-recently closed (i.e. the group
906with the rightmost closing parenthesis) of the last successful search
907pattern.
908
909This is primarily used inside C<(?{...})> blocks for examining text
910recently matched. For example, to effectively capture text to a variable
911(in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
912
0b9346e6 913 (?:(...)(?{ $var = $^N }))
b0c22438 914
915By setting and then using C<$var> in this way relieves you from having to
916worry about exactly which numbered set of parentheses they are.
917
918This variable was added in Perl 5.8.
919
920Mnemonic: the (possibly) Nested parenthesis that most recently closed.
921
922=item @LAST_MATCH_END
923
924=item @+
925X<@+> X<@LAST_MATCH_END>
926
927This array holds the offsets of the ends of the last successful
928submatches in the currently active dynamic scope. C<$+[0]> is
929the offset into the string of the end of the entire match. This
930is the same value as what the C<pos> function returns when called
931on the variable that was matched against. The I<n>th element
932of this array holds the offset of the I<n>th submatch, so
933C<$+[1]> is the offset past where C<$1> ends, C<$+[2]> the offset
7333b1c4 934past where C<$2> ends, and so on. You can use C<$#+> to determine
b0c22438 935how many subgroups were in the last successful match. See the
936examples given for the C<@-> variable.
937
938This variable was added in Perl 5.6.
939
940=item %LAST_PAREN_MATCH
941
942=item %+
5b442a2a 943X<%+> X<%LAST_PAREN_MATCH>
b0c22438 944
945Similar to C<@+>, the C<%+> hash allows access to the named capture
946buffers, should they exist, in the last successful match in the
947currently active dynamic scope.
948
949For example, C<$+{foo}> is equivalent to C<$1> after the following match:
950
0b9346e6 951 'foo' =~ /(?<foo>foo)/;
b0c22438 952
953The keys of the C<%+> hash list only the names of buffers that have
954captured (and that are thus associated to defined values).
955
956The underlying behaviour of C<%+> is provided by the
957L<Tie::Hash::NamedCapture> module.
958
959B<Note:> C<%-> and C<%+> are tied views into a common internal hash
960associated with the last successful regular expression. Therefore mixing
961iterative access to them via C<each> may have unpredictable results.
962Likewise, if the last successful match changes, then the results may be
963surprising.
964
965This variable was added in Perl 5.10.
a0d0e21e 966
b0c22438 967This variable is read-only and dynamically-scoped.
968
969=item @LAST_MATCH_START
970
971=item @-
972X<@-> X<@LAST_MATCH_START>
973
974C<$-[0]> is the offset of the start of the last successful match.
975C<$-[>I<n>C<]> is the offset of the start of the substring matched by
976I<n>-th subpattern, or undef if the subpattern did not match.
977
978Thus, after a match against C<$_>, C<$&> coincides with C<substr $_, $-[0],
979$+[0] - $-[0]>. Similarly, $I<n> coincides with C<substr $_, $-[n],
980$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
981C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>. One can use C<$#-> to find the last
982matched subgroup in the last successful match. Contrast with
983C<$#+>, the number of subgroups in the regular expression. Compare
984with C<@+>.
985
986This array holds the offsets of the beginnings of the last
987successful submatches in the currently active dynamic scope.
988C<$-[0]> is the offset into the string of the beginning of the
7333b1c4 989entire match. The I<n>th element of this array holds the offset
b0c22438 990of the I<n>th submatch, so C<$-[1]> is the offset where C<$1>
991begins, C<$-[2]> the offset where C<$2> begins, and so on.
992
993After a match against some variable C<$var>:
994
995=over 5
996
997=item C<$`> is the same as C<substr($var, 0, $-[0])>
998
999=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
1000
1001=item C<$'> is the same as C<substr($var, $+[0])>
1002
1003=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
1004
1005=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
1006
1007=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>
1008
1009=back
1010
1011This variable was added in Perl 5.6.
1012
5b442a2a 1013=item %LAST_MATCH_START
1014
b0c22438 1015=item %-
5b442a2a 1016X<%-> X<%LAST_MATCH_START>
b0c22438 1017
1018Similar to C<%+>, this variable allows access to the named capture groups
1019in the last successful match in the currently active dynamic scope. To
1020each capture group name found in the regular expression, it associates a
1021reference to an array containing the list of values captured by all
1022buffers with that name (should there be several of them), in the order
1023where they appear.
1024
1025Here's an example:
1026
1027 if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
1028 foreach my $bufname (sort keys %-) {
1029 my $ary = $-{$bufname};
1030 foreach my $idx (0..$#$ary) {
1031 print "\$-{$bufname}[$idx] : ",
1032 (defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
1033 "\n";
1034 }
1035 }
1036 }
1037
1038would print out:
1039
0b9346e6 1040 $-{A}[0] : '1'
1041 $-{A}[1] : '3'
1042 $-{B}[0] : '2'
1043 $-{B}[1] : '4'
b0c22438 1044
1045The keys of the C<%-> hash correspond to all buffer names found in
1046the regular expression.
1047
1048The behaviour of C<%-> is implemented via the
1049L<Tie::Hash::NamedCapture> module.
1050
1051B<Note:> C<%-> and C<%+> are tied views into a common internal hash
1052associated with the last successful regular expression. Therefore mixing
1053iterative access to them via C<each> may have unpredictable results.
1054Likewise, if the last successful match changes, then the results may be
1055surprising.
1056
1057This variable was added in Perl 5.10
1058
1059This variable is read-only and dynamically-scoped.
1060
1061=item $LAST_REGEXP_CODE_RESULT
1062
1063=item $^R
1064X<$^R> X<$LAST_REGEXP_CODE_RESULT>
1065
1066The result of evaluation of the last successful C<(?{ code })>
1067regular expression assertion (see L<perlre>). May be written to.
1068
1069This variable was added in Perl 5.005.
a0d0e21e 1070
a3621e74 1071=item ${^RE_DEBUG_FLAGS}
ca1b95ae 1072X<${^RE_DEBUG_FLAGS}>
a3621e74
YO
1073
1074The current value of the regex debugging flags. Set to 0 for no debug output
b0c22438 1075even when the C<re 'debug'> module is loaded. See L<re> for details.
1076
1077This variable was added in Perl 5.10.
a3621e74 1078
0111c4fd 1079=item ${^RE_TRIE_MAXBUF}
ca1b95ae 1080X<${^RE_TRIE_MAXBUF}>
a3621e74
YO
1081
1082Controls how certain regex optimisations are applied and how much memory they
1083utilize. This value by default is 65536 which corresponds to a 512kB temporary
1084cache. Set this to a higher value to trade memory for speed when matching
1085large alternations. Set it to a lower value if you want the optimisations to
1086be as conservative of memory as possible but still occur, and set it to a
1087negative value to prevent the optimisation and conserve the most memory.
1088Under normal situations this variable should be of no interest to you.
1089
b0c22438 1090This variable was added in Perl 5.10.
a0d0e21e 1091
b0c22438 1092=back
a0d0e21e 1093
b0c22438 1094=head2 Variables related to filehandles
a0d0e21e 1095
b0c22438 1096Variables that depend on the currently selected filehandle may be set
1097by calling an appropriate object method on the C<IO::Handle> object,
1098although this is less efficient than using the regular built-in
1099variables. (Summary lines below for this contain the word HANDLE.)
1100First you must say
6e2995f4 1101
0b9346e6 1102 use IO::Handle;
0462a1ab 1103
b0c22438 1104after which you may use either
0462a1ab 1105
0b9346e6 1106 method HANDLE EXPR
0462a1ab 1107
b0c22438 1108or more safely,
0462a1ab 1109
0b9346e6 1110 HANDLE->method(EXPR)
0462a1ab 1111
b0c22438 1112Each method returns the old value of the C<IO::Handle> attribute. The
1113methods each take an optional EXPR, which, if supplied, specifies the
1114new value for the C<IO::Handle> attribute in question. If not
1115supplied, most methods do nothing to the current value--except for
1116C<autoflush()>, which will assume a 1 for you, just to be different.
0462a1ab 1117
b0c22438 1118Because loading in the C<IO::Handle> class is an expensive operation,
1119you should learn how to use the regular built-in variables.
1120
1121A few of these variables are considered "read-only". This means that
1122if you try to assign to this variable, either directly or indirectly
1123through a reference, you'll raise a run-time exception.
1124
1125You should be very careful when modifying the default values of most
1126special variables described in this document. In most cases you want
1127to localize these variables before changing them, since if you don't,
1128the change may affect other modules which rely on the default values
1129of the special variables that you have changed. This is one of the
1130correct ways to read the whole file at once:
1131
0b9346e6 1132 open my $fh, "<", "foo" or die $!;
1133 local $/; # enable localized slurp mode
1134 my $content = <$fh>;
1135 close $fh;
b0c22438 1136
1137But the following code is quite bad:
1138
0b9346e6 1139 open my $fh, "<", "foo" or die $!;
1140 undef $/; # enable slurp mode
1141 my $content = <$fh>;
1142 close $fh;
b0c22438 1143
1144since some other module, may want to read data from some file in the
1145default "line mode", so if the code we have just presented has been
1146executed, the global value of C<$/> is now changed for any other code
1147running inside the same Perl interpreter.
1148
1149Usually when a variable is localized you want to make sure that this
1150change affects the shortest scope possible. So unless you are already
1151inside some short C<{}> block, you should create one yourself. For
1152example:
1153
0b9346e6 1154 my $content = '';
1155 open my $fh, "<", "foo" or die $!;
1156 {
1157 local $/;
1158 $content = <$fh>;
1159 }
1160 close $fh;
0462a1ab 1161
b0c22438 1162Here is an example of how your own code can go broken:
0462a1ab 1163
0b9346e6 1164 for ( 1..3 ){
1165 $\ = "\r\n";
1166 nasty_break();
1167 print "$_";
1168 }
1169
1170 sub nasty_break {
1171 $\ = "\f";
1172 # do something with $_
1173 }
0462a1ab 1174
0b9346e6 1175You probably expect this code to print the equivalent of
0462a1ab 1176
0b9346e6 1177 "1\r\n2\r\n3\r\n"
0462a1ab 1178
b0c22438 1179but instead you get:
0462a1ab 1180
0b9346e6 1181 "1\f2\f3\f"
0462a1ab 1182
0b9346e6 1183Why? Because C<nasty_break()> modifies C<$\> without localizing it
1184first. The value you set in C<nasty_break()> is still there when you
1185return. The fix is to add C<local()> so the value doesn't leak out of
1186C<nasty_break()>:
6e2995f4 1187
0b9346e6 1188 local $\ = "\f";
a0d0e21e 1189
b0c22438 1190It's easy to notice the problem in such a short example, but in more
1191complicated code you are looking for trouble if you don't localize
1192changes to the special variables.
a0d0e21e 1193
b0c22438 1194=over 8
a0d0e21e 1195
b0c22438 1196=item $ARGV
1197X<$ARGV>
fb73857a 1198
ca1b95ae 1199Contains the name of the current file when reading from C<< <> >>.
b0c22438 1200
1201=item @ARGV
1202X<@ARGV>
1203
ca1b95ae 1204The array C<@ARGV> contains the command-line arguments intended for
b0c22438 1205the script. C<$#ARGV> is generally the number of arguments minus
1206one, because C<$ARGV[0]> is the first argument, I<not> the program's
57f6eff5 1207command name itself. See L</$0> for the command name.
b0c22438 1208
84dabc03 1209=item ARGV
1210X<ARGV>
1211
1212The special filehandle that iterates over command-line filenames in
1213C<@ARGV>. Usually written as the null filehandle in the angle operator
1214C<< <> >>. Note that currently C<ARGV> only has its magical effect
1215within the C<< <> >> operator; elsewhere it is just a plain filehandle
1216corresponding to the last file opened by C<< <> >>. In particular,
1217passing C<\*ARGV> as a parameter to a function that expects a filehandle
1218may not cause your function to automatically read the contents of all the
1219files in C<@ARGV>.
1220
b0c22438 1221=item ARGVOUT
1222X<ARGVOUT>
1223
1224The special filehandle that points to the currently open output file
1225when doing edit-in-place processing with B<-i>. Useful when you have
1226to do a lot of inserting and don't want to keep modifying C<$_>. See
1227L<perlrun> for the B<-i> switch.
1228
5b442a2a 1229=item Handle->output_field_separator( EXPR )
84dabc03 1230
1231=item $OUTPUT_FIELD_SEPARATOR
1232
1233=item $OFS
1234
1235=item $,
1236X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>
1237
1238The output field separator for the print operator. If defined, this
1239value is printed between each of print's arguments. Default is C<undef>.
1240
1241Mnemonic: what is printed when there is a "," in your print statement.
1242
5b442a2a 1243=item HANDLE->input_line_number( EXPR )
b0c22438 1244
1245=item $INPUT_LINE_NUMBER
1246
1247=item $NR
1248
1249=item $.
1250X<$.> X<$NR> X<$INPUT_LINE_NUMBER> X<line number>
1251
1252Current line number for the last filehandle accessed.
1253
1254Each filehandle in Perl counts the number of lines that have been read
7333b1c4 1255from it. (Depending on the value of C<$/>, Perl's idea of what
b0c22438 1256constitutes a line may not match yours.) When a line is read from a
1257filehandle (via C<readline()> or C<< <> >>), or when C<tell()> or
1258C<seek()> is called on it, C<$.> becomes an alias to the line counter
1259for that filehandle.
1260
1261You can adjust the counter by assigning to C<$.>, but this will not
1262actually move the seek pointer. I<Localizing C<$.> will not localize
1263the filehandle's line count>. Instead, it will localize perl's notion
1264of which filehandle C<$.> is currently aliased to.
1265
1266C<$.> is reset when the filehandle is closed, but B<not> when an open
1267filehandle is reopened without an intervening C<close()>. For more
1268details, see L<perlop/"IE<sol>O Operators">. Because C<< <> >> never does
1269an explicit close, line numbers increase across C<ARGV> files (but see
1270examples in L<perlfunc/eof>).
1271
1272You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
1273line counter for a given filehandle without having to worry about
1274which handle you last accessed.
1275
1276Mnemonic: many programs use "." to mean the current line number.
1277
5b442a2a 1278=item HANDLE->input_record_separator( EXPR )
b0c22438 1279
1280=item $INPUT_RECORD_SEPARATOR
1281
1282=item $RS
1283
1284=item $/
1285X<$/> X<$RS> X<$INPUT_RECORD_SEPARATOR>
1286
84dabc03 1287The input record separator, newline by default. This influences Perl's
7333b1c4 1288idea of what a "line" is. Works like B<awk>'s RS variable, including
84dabc03 1289treating empty lines as a terminator if set to the null string (an
1290empty line cannot contain any spaces or tabs). You may set it to a
1291multi-character string to match a multi-character terminator, or to
1292C<undef> to read through the end of file. Setting it to C<"\n\n">
1293means something slightly different than setting to C<"">, if the file
1294contains consecutive empty lines. Setting to C<""> will treat two or
1295more consecutive empty lines as a single empty line. Setting to
1296C<"\n\n"> will blindly assume that the next input character belongs to
1297the next paragraph, even if it's a newline.
b0c22438 1298
1299 local $/; # enable "slurp" mode
1300 local $_ = <FH>; # whole file now here
1301 s/\n[ \t]+/ /g;
1302
7333b1c4 1303Remember: the value of C<$/> is a string, not a regex. B<awk> has to
b0c22438 1304be better for something. :-)
1305
1306Setting C<$/> to a reference to an integer, scalar containing an
1307integer, or scalar that's convertible to an integer will attempt to
1308read records instead of lines, with the maximum record size being the
1309referenced integer. So this:
1310
1311 local $/ = \32768; # or \"32768", or \$var_containing_32768
1312 open my $fh, "<", $myfile or die $!;
1313 local $_ = <$fh>;
fb73857a 1314
7333b1c4 1315will read a record of no more than 32768 bytes from FILE. If you're
b0c22438 1316not reading from a record-oriented file (or your OS doesn't have
1317record-oriented files), then you'll likely get a full chunk of data
7333b1c4 1318with every read. If a record is larger than the record size you've
1319set, you'll get the record back in pieces. Trying to set the record
b0c22438 1320size to zero or less will cause reading in the (rest of the) whole file.
6e2995f4 1321
b0c22438 1322On VMS, record reads are done with the equivalent of C<sysread>,
1323so it's best not to mix record and non-record reads on the same
5b442a2a 1324file. (This is unlikely to be a problem, because any file you'd
b0c22438 1325want to read in record mode is probably unusable in line mode.)
1326Non-VMS systems do normal I/O, so it's safe to mix record and
1327non-record reads of a file.
5c055ba3 1328
57f6eff5 1329See also L<perlport/"Newlines">. Also see L</$.>.
9bf22702 1330
b0c22438 1331Mnemonic: / delimits line boundaries when quoting poetry.
5c055ba3 1332
5b442a2a 1333=item Handle->output_record_separator( EXPR )
84902520 1334
b0c22438 1335=item $OUTPUT_RECORD_SEPARATOR
84902520 1336
b0c22438 1337=item $ORS
84902520 1338
b0c22438 1339=item $\
1340X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>
84902520 1341
b0c22438 1342The output record separator for the print operator. If defined, this
1343value is printed after the last of print's arguments. Default is C<undef>.
84902520 1344
b0c22438 1345Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
1346Also, it's just like C<$/>, but it's what you get "back" from Perl.
84902520 1347
5b442a2a 1348=item HANDLE->autoflush( EXPR )
1349
1350=item $OUTPUT_AUTOFLUSH
1351
84dabc03 1352=item $|
1353X<$|> X<autoflush> X<flush> X<$OUTPUT_AUTOFLUSH>
84902520 1354
84dabc03 1355If set to nonzero, forces a flush right away and after every write or
7333b1c4 1356print on the currently selected output channel. Default is 0
84dabc03 1357(regardless of whether the channel is really buffered by the system or
1358not; C<$|> tells you only whether you've asked Perl explicitly to
1359flush after each write). STDOUT will typically be line buffered if
5b442a2a 1360output is to the terminal and block buffered otherwise. Setting this
84dabc03 1361variable is useful primarily when you are outputting to a pipe or
1362socket, such as when you are running a Perl program under B<rsh> and
5b442a2a 1363want to see the output as it's happening. This has no effect on input
c003e62a 1364buffering. See L<perlfunc/getc> for that. See L<perlfunc/select> on
84dabc03 1365how to select the output channel. See also L<IO::Handle>.
1366
1367Mnemonic: when you want your pipes to be piping hot.
1368
1369=back
84902520 1370
b0c22438 1371=head3 Variables related to formats
83ee9e09 1372
b0c22438 1373The special variables for formats are a subset of those for
69b55ccc 1374filehandles. See L<perlform> for more information about Perl's
1375formats.
83ee9e09 1376
b0c22438 1377=over 8
83ee9e09 1378
84dabc03 1379=item $ACCUMULATOR
1380
1381=item $^A
1382X<$^A> X<$ACCUMULATOR>
1383
1384The current value of the C<write()> accumulator for C<format()> lines.
1385A format contains C<formline()> calls that put their result into
7333b1c4 1386C<$^A>. After calling its format, C<write()> prints out the contents
84dabc03 1387of C<$^A> and empties. So you never really see the contents of C<$^A>
1388unless you call C<formline()> yourself and then look at it. See
96090e4f 1389L<perlform> and L<perlfunc/"formline PICTURE,LIST">.
84dabc03 1390
5b442a2a 1391=item HANDLE->format_formfeed(EXPR)
1392
1393=item $FORMAT_FORMFEED
1394
84dabc03 1395=item $^L
1396X<$^L> X<$FORMAT_FORMFEED>
1397
1398What formats output as a form feed. The default is C<\f>.
1399
b0c22438 1400=item HANDLE->format_page_number(EXPR)
83ee9e09 1401
b0c22438 1402=item $FORMAT_PAGE_NUMBER
83ee9e09 1403
b0c22438 1404=item $%
1405X<$%> X<$FORMAT_PAGE_NUMBER>
83ee9e09 1406
b0c22438 1407The current page number of the currently selected output channel.
83ee9e09 1408
b0c22438 1409Mnemonic: C<%> is page number in B<nroff>.
7619c85e 1410
b0c22438 1411=item HANDLE->format_lines_left(EXPR)
b9ac3b5b 1412
b0c22438 1413=item $FORMAT_LINES_LEFT
66558a10 1414
b0c22438 1415=item $-
1416X<$-> X<$FORMAT_LINES_LEFT>
fb73857a 1417
b0c22438 1418The number of lines left on the page of the currently selected output
1419channel.
fa05a9fd 1420
b0c22438 1421Mnemonic: lines_on_page - lines_printed.
fa05a9fd 1422
84dabc03 1423=item Handle->format_line_break_characters EXPR
fb73857a 1424
84dabc03 1425=item $FORMAT_LINE_BREAK_CHARACTERS
a0d0e21e 1426
84dabc03 1427=item $:
1428X<$:> X<FORMAT_LINE_BREAK_CHARACTERS>
a0d0e21e 1429
84dabc03 1430The current set of characters after which a string may be broken to
1431fill continuation fields (starting with C<^>) in a format. The default is
1432S<" \n-">, to break on a space, newline, or a hyphen.
a0d0e21e 1433
84dabc03 1434Mnemonic: a "colon" in poetry is a part of a line.
1435
1436=item HANDLE->format_lines_per_page(EXPR)
1437
1438=item $FORMAT_LINES_PER_PAGE
1439
1440=item $=
1441X<$=> X<$FORMAT_LINES_PER_PAGE>
1442
1443The current page length (printable lines) of the currently selected
1444output channel. The default is 60.
1445
1446Mnemonic: = has horizontal lines.
7c36658b 1447
b0c22438 1448=item HANDLE->format_top_name(EXPR)
7c36658b 1449
b0c22438 1450=item $FORMAT_TOP_NAME
a05d7ebb 1451
b0c22438 1452=item $^
1453X<$^> X<$FORMAT_TOP_NAME>
fde18df1 1454
b0c22438 1455The name of the current top-of-page format for the currently selected
1456output channel. The default is the name of the filehandle with C<_TOP>
1457appended. For example, the default format top name for the C<STDOUT>
12abbafd 1458filehandle is C<STDOUT_TOP>.
e07ea26a 1459
b0c22438 1460Mnemonic: points to top of page.
e07ea26a 1461
84dabc03 1462=item HANDLE->format_name(EXPR)
16070b82 1463
84dabc03 1464=item $FORMAT_NAME
aa2f2a36 1465
84dabc03 1466=item $~
1467X<$~> X<$FORMAT_NAME>
aa2f2a36 1468
84dabc03 1469The name of the current report format for the currently selected
1470output channel. The default format name is the same as the filehandle
1471name. For example, the default format name for the C<STDOUT>
1472filehandle is just C<STDOUT>.
16070b82 1473
84dabc03 1474Mnemonic: brother to C<$^>.
16070b82 1475
b0c22438 1476=back
a0d0e21e 1477
84dabc03 1478=head2 Error Variables
b0c22438 1479X<error> X<exception>
a0d0e21e 1480
b0c22438 1481The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1482about different types of error conditions that may appear during
1483execution of a Perl program. The variables are shown ordered by
1484the "distance" between the subsystem which reported the error and
1485the Perl process. They correspond to errors detected by the Perl
1486interpreter, C library, operating system, or an external program,
1487respectively.
4438c4b7 1488
b0c22438 1489To illustrate the differences between these variables, consider the
7fd683ff 1490following Perl expression, which uses a single-quoted string. After
1491execution of this statement, perl may have set all four special error
7333b1c4 1492variables:
4438c4b7 1493
ca1b95ae 1494 eval q{
7333b1c4 1495 open my $pipe, "/cdrom/install |" or die $!;
1496 my @res = <$pipe>;
1497 close $pipe or die "bad pipe: $?, $!";
1498 };
a0d0e21e 1499
7333b1c4 1500When perl executes the C<eval()> expression, it translates the
1501C<open()>, C<< <PIPE> >>, and C<close> calls in the C run-time library
69b55ccc 1502and thence to the operating system kernel. perl sets C<$!> to
7333b1c4 1503the C library's C<errno> if one of these calls fails.
2a8c8378 1504
84dabc03 1505C<$@> is set if the string to be C<eval>-ed did not compile (this may
1506happen if C<open> or C<close> were imported with bad prototypes), or
7333b1c4 1507if Perl code executed during evaluation C<die()>d. In these cases the
0b9346e6 1508value of C<$@> is the compile error, or the argument to C<die> (which
84dabc03 1509will interpolate C<$!> and C<$?>). (See also L<Fatal>, though.)
2a8c8378 1510
84dabc03 1511Under a few operating systems, C<$^E> may contain a more verbose error
1512indicator, such as in this case, "CDROM tray not closed." Systems that
1513do not support extended error messages leave C<$^E> the same as C<$!>.
a0d0e21e 1514
b0c22438 1515Finally, C<$?> may be set to non-0 value if the external program
84dabc03 1516F</cdrom/install> fails. The upper eight bits reflect specific error
1517conditions encountered by the program (the program's C<exit()> value).
1518The lower eight bits reflect mode of failure, like signal death and
57f6eff5 1519core dump information. See L<wait(2)> for details. In contrast to
84dabc03 1520C<$!> and C<$^E>, which are set only if error condition is detected,
1521the variable C<$?> is set on each C<wait> or pipe C<close>,
1522overwriting the old value. This is more like C<$@>, which on every
1523C<eval()> is always set on failure and cleared on success.
a0d0e21e 1524
b0c22438 1525For more details, see the individual descriptions at C<$@>, C<$!>,
1526C<$^E>, and C<$?>.
38e4f4ae 1527
0b9346e6 1528=over 8
1529
b0c22438 1530=item ${^CHILD_ERROR_NATIVE}
1531X<$^CHILD_ERROR_NATIVE>
a0d0e21e 1532
b0c22438 1533The native status returned by the last pipe close, backtick (C<``>)
1534command, successful call to C<wait()> or C<waitpid()>, or from the
1535C<system()> operator. On POSIX-like systems this value can be decoded
1536with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED,
1537WSTOPSIG and WIFCONTINUED functions provided by the L<POSIX> module.
a0d0e21e 1538
b0c22438 1539Under VMS this reflects the actual VMS exit status; i.e. it is the
1540same as C<$?> when the pragma C<use vmsish 'status'> is in effect.
a0d0e21e 1541
b0c22438 1542This variable was added in Perl 5.8.9.
a0d0e21e 1543
5b442a2a 1544=item $EXTENDED_OS_ERROR
1545
84dabc03 1546=item $^E
1547X<$^E> X<$EXTENDED_OS_ERROR>
1548
1549Error information specific to the current operating system. At the
1550moment, this differs from C<$!> under only VMS, OS/2, and Win32 (and
1551for MacPerl). On all other platforms, C<$^E> is always just the same
1552as C<$!>.
1553
1554Under VMS, C<$^E> provides the VMS status value from the last system
1555error. This is more specific information about the last system error
1556than that provided by C<$!>. This is particularly important when C<$!>
1557is set to B<EVMSERR>.
1558
1559Under OS/2, C<$^E> is set to the error code of the last call to OS/2
1560API either via CRT, or directly from perl.
1561
1562Under Win32, C<$^E> always returns the last error information reported
1563by the Win32 call C<GetLastError()> which describes the last error
1564from within the Win32 API. Most Win32-specific code will report errors
1565via C<$^E>. ANSI C and Unix-like calls set C<errno> and so most
1566portable Perl code will report errors via C<$!>.
1567
1568Caveats mentioned in the description of C<$!> generally apply to
1569C<$^E>, also.
1570
1571This variable was added in Perl 5.003.
1572
1573Mnemonic: Extra error explanation.
0b9346e6 1574
84dabc03 1575=item $EXCEPTIONS_BEING_CAUGHT
1576
1577=item $^S
1578X<$^S> X<$EXCEPTIONS_BEING_CAUGHT>
1579
1580Current state of the interpreter.
1581
ca1b95ae 1582 $^S State
1583 --------- -------------------
1584 undef Parsing module/eval
1585 true (1) Executing an eval
1586 false (0) Otherwise
84dabc03 1587
1588The first state may happen in C<$SIG{__DIE__}> and C<$SIG{__WARN__}>
1589handlers.
1590
1591This variable was added in Perl 5.004.
1592
1593=item $WARNING
1594
1595=item $^W
1596X<$^W> X<$WARNING>
1597
1598The current value of the warning switch, initially true if B<-w> was
1599used, false otherwise, but directly modifiable.
1600
1601See also L<warnings>.
1602
0b9346e6 1603Mnemonic: related to the B<-w> switch.
84dabc03 1604
1605=item ${^WARNING_BITS}
ca1b95ae 1606X<${^WARNING_BITS}>
84dabc03 1607
1608The current set of warning checks enabled by the C<use warnings> pragma.
1609See the documentation of C<warnings> for more details.
1610
1611This variable was added in Perl 5.10.
1612
b0c22438 1613=item $OS_ERROR
5ccee41e 1614
b0c22438 1615=item $ERRNO
5ccee41e 1616
b0c22438 1617=item $!
1618X<$!> X<$ERRNO> X<$OS_ERROR>
9b0e6e7a 1619
a73bef78
JL
1620When referenced, C<$!> retrieves the current value
1621of the C C<errno> integer variable.
1622If C<$!> is assigned a numerical value, that value is stored in C<errno>.
1623When referenced as a string, C<$!> yields the system error string
1624corresponding to C<errno>.
1625
1626Many system or library calls set C<errno> if they fail,
1627to indicate the cause of failure. They usually do B<not>
1628set C<errno> to zero if they succeed. This means C<errno>,
1629hence C<$!>, is meaningful only I<immediately> after a B<failure>:
1630
1631 if (open my $fh, "<", $filename) {
ca1b95ae 1632 # Here $! is meaningless.
1633 ...
7fd683ff 1634 }
ca1b95ae 1635 else {
1636 # ONLY here is $! meaningful.
1637 ...
1638 # Already here $! might be meaningless.
b0c22438 1639 }
1640 # Since here we might have either success or failure,
a73bef78 1641 # $! is meaningless.
a0d0e21e 1642
a73bef78
JL
1643Here, I<meaningless> means that C<$!> may be unrelated to the outcome
1644of the C<open()> operator. Assignment to C<$!> is similarly ephemeral.
1645It can be used immediately before invoking the C<die()> operator,
1646to set the exit value, or to inspect the system error string
1647corresponding to error I<n>, or to restore C<$!> to a meaningful state.
d54b56d5 1648
b0c22438 1649Mnemonic: What just went bang?
314d39ce 1650
b0c22438 1651=item %OS_ERROR
fb73857a 1652
b0c22438 1653=item %ERRNO
fb73857a 1654
b0c22438 1655=item %!
5b442a2a 1656X<%!> X<%OS_ERROR> X<%ERRNO>
a0d0e21e 1657
b0c22438 1658Each element of C<%!> has a true value only if C<$!> is set to that
1659value. For example, C<$!{ENOENT}> is true if and only if the current
84dabc03 1660value of C<$!> is C<ENOENT>; that is, if the most recent error was "No
1661such file or directory" (or its moral equivalent: not all operating
1662systems give that exact error, and certainly not all languages). To
1663check if a particular key is meaningful on your system, use C<exists
1664$!{the_key}>; for a list of legal keys, use C<keys %!>. See L<Errno>
7333b1c4 1665for more information, and also see L</$!>.
a0d0e21e 1666
b0c22438 1667This variable was added in Perl 5.005.
44f0be63 1668
84dabc03 1669=item $CHILD_ERROR
b687b08b 1670
84dabc03 1671=item $?
1672X<$?> X<$CHILD_ERROR>
a0d0e21e 1673
84dabc03 1674The status returned by the last pipe close, backtick (C<``>) command,
1675successful call to C<wait()> or C<waitpid()>, or from the C<system()>
1676operator. This is just the 16-bit status word returned by the
1677traditional Unix C<wait()> system call (or else is made up to look
1678like it). Thus, the exit value of the subprocess is really (C<<< $? >>
16798 >>>), and C<$? & 127> gives which signal, if any, the process died
1680from, and C<$? & 128> reports whether there was a core dump.
a0d0e21e 1681
84dabc03 1682Additionally, if the C<h_errno> variable is supported in C, its value
1683is returned via C<$?> if any C<gethost*()> function fails.
b687b08b 1684
84dabc03 1685If you have installed a signal handler for C<SIGCHLD>, the
1686value of C<$?> will usually be wrong outside that handler.
a0d0e21e 1687
84dabc03 1688Inside an C<END> subroutine C<$?> contains the value that is going to be
1689given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
1690change the exit status of your program. For example:
a0d0e21e 1691
84dabc03 1692 END {
1693 $? = 1 if $? == 255; # die would make it 255
1694 }
a0d0e21e 1695
84dabc03 1696Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
1697actual VMS exit status, instead of the default emulation of POSIX
1698status; see L<perlvms/$?> for details.
1699
1700Mnemonic: similar to B<sh> and B<ksh>.
a0d0e21e 1701
b0c22438 1702=item $EVAL_ERROR
f648820c 1703
b0c22438 1704=item $@
1705X<$@> X<$EVAL_ERROR>
a0d0e21e 1706
0b9346e6 1707The Perl syntax error message from the last C<eval()> operator. If C<$@> is
1708the null string, the last C<eval()> parsed and executed correctly
b0c22438 1709(although the operations you invoked may have failed in the normal
1710fashion).
a0d0e21e 1711
b0c22438 1712Warning messages are not collected in this variable. You can, however,
1713set up a routine to process warnings by setting C<$SIG{__WARN__}> as
7333b1c4 1714described in L</%SIG>.
748a9306 1715
b0c22438 1716Mnemonic: Where was the syntax error "at"?
7f315d2e 1717
b0c22438 1718=back
7f315d2e 1719
1fa81471
DR
1720=head2 Variables related to the interpreter state
1721
1722These variables provide information about the current interpreter state.
1723
1724=over 8
1725
1726=item $COMPILING
1727
1728=item $^C
1729X<$^C> X<$COMPILING>
1730
1731The current value of the flag associated with the B<-c> switch.
1732Mainly of use with B<-MO=...> to allow code to alter its behavior
1733when being compiled, such as for example to C<AUTOLOAD> at compile
1734time rather than normal, deferred loading. Setting
1735C<$^C = 1> is similar to calling C<B::minus_c>.
1736
1737This variable was added in Perl 5.6.
1738
1739=item $DEBUGGING
1740
1741=item $^D
1742X<$^D> X<$DEBUGGING>
1743
1744The current value of the debugging flags. May be read or set. Like its
1745command-line equivalent, you can use numeric or symbolic values, eg
1746C<$^D = 10> or C<$^D = "st">.
1747
1748Mnemonic: value of B<-D> switch.
1749
1750=item ${^ENCODING}
1751X<${^ENCODING}>
1752
1753The I<object reference> to the C<Encode> object that is used to convert
1754the source code to Unicode. Thanks to this variable your Perl script
1755does not have to be written in UTF-8. Default is I<undef>. The direct
1756manipulation of this variable is highly discouraged.
1757
1758This variable was added in Perl 5.8.2.
1759
1760=item ${^GLOBAL_PHASE}
1761X<${^GLOBAL_PHASE}>
1762
1763The current phase of the perl interpreter.
1764
1765Possible values are:
1766
1767=over 8
1768
1769=item CONSTRUCT
1770
1771The C<PerlInterpreter*> is being constructed via C<perl_construct>. This
1772value is mostly there for completeness and for use via the
1773underlying C variable C<PL_phase>. It's not really possible for Perl
1774code to be executed unless construction of the interpreter is
1775finished.
1776
1777=item START
1778
1779This is the global compile-time. That includes, basically, every
1780C<BEGIN> block executed directly or indirectly from during the
1781compile-time of the top-level program.
1782
1783This phase is not called "BEGIN" to avoid confusion with
1784C<BEGIN>-blocks, as those are executed during compile-time of any
1785compilation unit, not just the top-level program. A new, localised
1786compile-time entered at run-time, for example by constructs as
1787C<eval "use SomeModule"> are not global interpreter phases, and
1788therefore aren't reflected by C<${^GLOBAL_PHASE}>.
1789
1790=item CHECK
1791
1792Execution of any C<CHECK> blocks.
1793
1794=item INIT
1795
1796Similar to "CHECK", but for C<INIT>-blocks, not C<CHECK> blocks.
1797
1798=item RUN
1799
1800The main run-time, i.e. the execution of C<PL_main_root>.
1801
1802=item END
1803
1804Execution of any C<END> blocks.
1805
1806=item DESTRUCT
1807
1808Global destruction.
1809
1810=back
1811
1812Also note that there's no value for UNITCHECK-blocks. That's because
1813those are run for each compilation unit individually, and therefore is
1814not a global interpreter phase.
1815
1816Not every program has to go through each of the possible phases, but
1817transition from one phase to another can only happen in the order
1818described in the above list.
1819
1820An example of all of the phases Perl code can see:
1821
1822 BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }
1823
1824 INIT { print "init-time: ${^GLOBAL_PHASE}\n" }
1825
1826 CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }
1827
1828 {
1829 package Print::Phase;
1830
1831 sub new {
1832 my ($class, $time) = @_;
1833 return bless \$time, $class;
1834 }
1835
1836 sub DESTROY {
1837 my $self = shift;
1838 print "$$self: ${^GLOBAL_PHASE}\n";
1839 }
1840 }
1841
1842 print "run-time: ${^GLOBAL_PHASE}\n";
1843
1844 my $runtime = Print::Phase->new(
1845 "lexical variables are garbage collected before END"
1846 );
1847
1848 END { print "end-time: ${^GLOBAL_PHASE}\n" }
1849
1850 our $destruct = Print::Phase->new(
1851 "package variables are garbage collected after END"
1852 );
1853
1854This will print out
1855
1856 compile-time: START
1857 check-time: CHECK
1858 init-time: INIT
1859 run-time: RUN
1860 lexical variables are garbage collected before END: RUN
1861 end-time: END
1862 package variables are garbage collected after END: DESTRUCT
1863
1864This variable was added in Perl 5.14.0.
1865
1866=item $^H
1867X<$^H>
1868
1869WARNING: This variable is strictly for internal use only. Its availability,
1870behavior, and contents are subject to change without notice.
1871
1872This variable contains compile-time hints for the Perl interpreter. At the
1873end of compilation of a BLOCK the value of this variable is restored to the
1874value when the interpreter started to compile the BLOCK.
1875
1876When perl begins to parse any block construct that provides a lexical scope
1877(e.g., eval body, required file, subroutine body, loop body, or conditional
1878block), the existing value of C<$^H> is saved, but its value is left unchanged.
1879When the compilation of the block is completed, it regains the saved value.
1880Between the points where its value is saved and restored, code that
1881executes within BEGIN blocks is free to change the value of C<$^H>.
1882
1883This behavior provides the semantic of lexical scoping, and is used in,
1884for instance, the C<use strict> pragma.
1885
1886The contents should be an integer; different bits of it are used for
1887different pragmatic flags. Here's an example:
1888
1889 sub add_100 { $^H |= 0x100 }
1890
1891 sub foo {
1892 BEGIN { add_100() }
1893 bar->baz($boon);
1894 }
1895
1896Consider what happens during execution of the BEGIN block. At this point
1897the BEGIN block has already been compiled, but the body of C<foo()> is still
1898being compiled. The new value of C<$^H> will therefore be visible only while
1899the body of C<foo()> is being compiled.
1900
1901Substitution of C<BEGIN { add_100() }> block with:
1902
1903 BEGIN { require strict; strict->import('vars') }
1904
1905demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
1906version of the same lexical pragma:
1907
1908 BEGIN { require strict; strict->import('vars') if $condition }
1909
1910This variable was added in Perl 5.003.
1911
1912=item %^H
1913X<%^H>
1914
1915The C<%^H> hash provides the same scoping semantic as C<$^H>. This makes it
1916useful for implementation of lexically scoped pragmas. See L<perlpragma>.
1917
1918When putting items into C<%^H>, in order to avoid conflicting with other
1919users of the hash there is a convention regarding which keys to use.
1920A module should use only keys that begin with the module's name (the
1921name of its main package) and a "/" character. For example, a module
1922C<Foo::Bar> should use keys such as C<Foo::Bar/baz>.
1923
1924This variable was added in Perl 5.6.
1925
1926=item ${^OPEN}
1927X<${^OPEN}>
1928
1929An internal variable used by PerlIO. A string in two parts, separated
1930by a C<\0> byte, the first part describes the input layers, the second
1931part describes the output layers.
1932
1933This variable was added in Perl 5.8.0.
1934
1935=item $PERLDB
1936
1937=item $^P
1938X<$^P> X<$PERLDB>
1939
1940The internal variable for debugging support. The meanings of the
1941various bits are subject to change, but currently indicate:
1942
1943=over 6
1944
1945=item 0x01
1946
1947Debug subroutine enter/exit.
1948
1949=item 0x02
1950
1951Line-by-line debugging. Causes C<DB::DB()> subroutine to be called for each
1952statement executed. Also causes saving source code lines (like 0x400).
1953
1954=item 0x04
1955
1956Switch off optimizations.
1957
1958=item 0x08
1959
1960Preserve more data for future interactive inspections.
1961
1962=item 0x10
1963
1964Keep info about source lines on which a subroutine is defined.
1965
1966=item 0x20
1967
1968Start with single-step on.
1969
1970=item 0x40
1971
1972Use subroutine address instead of name when reporting.
1973
1974=item 0x80
1975
1976Report C<goto &subroutine> as well.
1977
1978=item 0x100
1979
1980Provide informative "file" names for evals based on the place they were compiled.
1981
1982=item 0x200
1983
1984Provide informative names to anonymous subroutines based on the place they
1985were compiled.
1986
1987=item 0x400
1988
1989Save source code lines into C<@{"_<$filename"}>.
1990
1991=back
1992
1993Some bits may be relevant at compile-time only, some at
1994run-time only. This is a new mechanism and the details may change.
1995See also L<perldebguts>.
1996
1997=item ${^TAINT}
1998X<${^TAINT}>
1999
2000Reflects if taint mode is on or off. 1 for on (the program was run with
2001B<-T>), 0 for off, -1 when only taint warnings are enabled (i.e. with
2002B<-t> or B<-TU>).
2003
2004This variable is read-only.
2005
2006This variable was added in Perl 5.8.
2007
2008=item ${^UNICODE}
2009X<${^UNICODE}>
2010
2011Reflects certain Unicode settings of Perl. See L<perlrun>
2012documentation for the C<-C> switch for more information about
2013the possible values.
2014
2015This variable is set during Perl startup and is thereafter read-only.
2016
2017This variable was added in Perl 5.8.2.
2018
2019=item ${^UTF8CACHE}
2020X<${^UTF8CACHE}>
2021
2022This variable controls the state of the internal UTF-8 offset caching code.
20231 for on (the default), 0 for off, -1 to debug the caching code by checking
2024all its results against linear scans, and panicking on any discrepancy.
2025
2026This variable was added in Perl 5.8.9.
2027
2028=item ${^UTF8LOCALE}
2029X<${^UTF8LOCALE}>
2030
2031This variable indicates whether a UTF-8 locale was detected by perl at
2032startup. This information is used by perl when it's in
2033adjust-utf8ness-to-locale mode (as when run with the C<-CL> command-line
2034switch); see L<perlrun> for more info on this.
2035
2036This variable was added in Perl 5.8.8.
2037
2038=back
2039
b0c22438 2040=head2 Deprecated and removed variables
7f315d2e 2041
0b9346e6 2042Deprecating a variable announces the intent of the perl maintainers to
1c2e8cca 2043eventually remove the variable from the language. It may still be
b0c22438 2044available despite its status. Using a deprecated variable triggers
2045a warning.
7f315d2e 2046
84dabc03 2047Once a variable is removed, its use triggers an error telling you
b0c22438 2048the variable is unsupported.
7f315d2e 2049
84dabc03 2050See L<perldiag> for details about error messages.
7f315d2e 2051
b0c22438 2052=over 8
7f315d2e 2053
5b442a2a 2054=item $OFMT
2055
84dabc03 2056=item $#
5b442a2a 2057X<$#> X<$OFMT>
84dabc03 2058
38e5787b 2059C<$#> was a variable that could be used to format printed numbers.
84dabc03 2060After a deprecation cycle, its magic was removed in Perl 5.10 and
2061using it now triggers a warning: C<$# is no longer supported>.
2062
2063This is not the sigil you use in front of an array name to get the
2064last index, like C<$#array>. That's still how you get the last index
2065of an array in Perl. The two have nothing to do with each other.
2066
2067Deprecated in Perl 5.
2068
2069Removed in Perl 5.10.
2070
7f315d2e
CO
2071=item $*
2072X<$*>
2073
84dabc03 2074C<$*> was a variable that you could use to enable multiline matching.
7f315d2e
CO
2075After a deprecation cycle, its magic was removed in Perl 5.10.
2076Using it now triggers a warning: C<$* is no longer supported>.
84dabc03 2077You should use the C</s> and C</m> regexp modifiers instead.
7f315d2e 2078
b0c22438 2079Deprecated in Perl 5.
7f315d2e 2080
b0c22438 2081Removed in Perl 5.10.
7f315d2e 2082
5b442a2a 2083=item $ARRAY_BASE
2084
84dabc03 2085=item $[
5b442a2a 2086X<$[> X<$ARRAY_BASE>
84dabc03 2087
e1dccc0d
Z
2088C<$[> was a variable that you could use to offset the indexing of arrays
2089and strings. After a deprecation cycle, the feature was removed in
2090Perl 5.16. Two old ways of coping with the variability of the index
2091offset, which were rendered obsolete in Perl 5.000 when C<$[> became
2092effectively lexically scoped, are still supported: you can read it
2093(always yielding zero) and you can assign zero to it.
84dabc03 2094
0b9346e6 2095Deprecated in Perl 5.12.
84dabc03 2096
e1dccc0d
Z
2097Removed in Perl 5.16.
2098
5b442a2a 2099=item $OLD_PERL_VERSION
2100
b0c22438 2101=item $]
5b442a2a 2102X<$]> X<$OLD_PERL_VERSION>
55602bd2 2103
57f6eff5 2104See L</$^V> for a more modern representation of the Perl version that allows
d4ba9bf2 2105accurate string comparisons.
2106
b0c22438 2107The version + patchlevel / 1000 of the Perl interpreter. This variable
2108can be used to determine whether the Perl interpreter executing a
2109script is in the right range of versions:
55602bd2 2110
b0c22438 2111 warn "No checksumming!\n" if $] < 3.019;
55602bd2 2112
d4ba9bf2 2113The floating point representation can sometimes lead to inaccurate
2114numeric comparisons.
2115
b0c22438 2116See also the documentation of C<use VERSION> and C<require VERSION>
2117for a convenient way to fail if the running Perl interpreter is too old.
55602bd2 2118
b0c22438 2119Mnemonic: Is this version of perl in the right bracket?
19799a22 2120
b0c22438 2121Deprecated in Perl 5.6.
19799a22 2122
b0c22438 2123=back
2b92dfce 2124
0b9346e6 2125=cut