3 perlvar - Perl predefined variables
7 =head2 Predefined Names
9 The following names have special meaning to Perl. Most
10 punctuation names have reasonable mnemonics, or analogs in the
11 shells. Nevertheless, if you wish to use long variable names,
16 at the top of your program. This will alias all the short names to the
17 long names in the current package. Some even have medium names,
18 generally borrowed from B<awk>.
20 If you don't mind the performance hit, variables that depend on the
21 currently selected filehandle may instead be set by calling an
22 appropriate object method on the IO::Handle object. (Summary lines
23 below for this contain the word HANDLE.) First you must say
27 after which you may use either
35 Each method returns the old value of the IO::Handle attribute.
36 The methods each take an optional EXPR, which if supplied specifies the
37 new value for the IO::Handle attribute in question. If not supplied,
38 most methods do nothing to the current value--except for
39 autoflush(), which will assume a 1 for you, just to be different.
40 Because loading in the IO::Handle class is an expensive operation, you should
41 learn how to use the regular built-in variables.
43 A few of these variables are considered "read-only". This means that if
44 you try to assign to this variable, either directly or indirectly through
45 a reference, you'll raise a run-time exception.
47 The following list is ordered by scalar variables first, then the
48 arrays, then the hashes.
56 The default input and pattern-searching space. The following pairs are
59 while (<>) {...} # equivalent only in while!
60 while (defined($_ = <>)) {...}
71 Here are the places where Perl will assume $_ even if you
78 Various unary functions, including functions like ord() and int(), as well
79 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
84 Various list functions like print() and unlink().
88 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
89 without an C<=~> operator.
93 The default iterator variable in a C<foreach> loop if no other
98 The implicit iterator variable in the grep() and map() functions.
102 The default place to put an input record when a C<E<lt>FHE<gt>>
103 operation's result is tested by itself as the sole criterion of a C<while>
104 test. Outside a C<while> test, this will not happen.
108 (Mnemonic: underline is understood in certain operations.)
114 =item $E<lt>I<digits>E<gt>
116 Contains the subpattern from the corresponding set of capturing
117 parentheses from the last pattern match, not counting patterns
118 matched in nested blocks that have been exited already. (Mnemonic:
119 like \digits.) These variables are all read-only and dynamically
120 scoped to the current BLOCK.
126 The string matched by the last successful pattern match (not counting
127 any matches hidden within a BLOCK or eval() enclosed by the current
128 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
129 and dynamically scoped to the current BLOCK.
131 The use of this variable anywhere in a program imposes a considerable
132 performance penalty on all regular expression matches. See L<BUGS>.
138 The string preceding whatever was matched by the last successful
139 pattern match (not counting any matches hidden within a BLOCK or eval
140 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
141 string.) This variable is read-only.
143 The use of this variable anywhere in a program imposes a considerable
144 performance penalty on all regular expression matches. See L<BUGS>.
150 The string following whatever was matched by the last successful
151 pattern match (not counting any matches hidden within a BLOCK or eval()
152 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
157 print "$`:$&:$'\n"; # prints abc:def:ghi
159 This variable is read-only and dynamically scoped to the current BLOCK.
161 The use of this variable anywhere in a program imposes a considerable
162 performance penalty on all regular expression matches. See L<BUGS>.
164 =item $LAST_PAREN_MATCH
168 The last bracket matched by the last search pattern. This is useful if
169 you don't know which one of a set of alternative patterns matched. For
172 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
174 (Mnemonic: be positive and forward looking.)
175 This variable is read-only and dynamically scoped to the current BLOCK.
179 $+[0] is the offset of the end of the last successful match.
180 C<$+[>I<n>C<]> is the offset of the end of the substring matched by
181 I<n>-th subpattern, or undef if the subpattern did not match.
183 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
184 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
185 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
186 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#+> to find the number
187 of subgroups in the last successful match. Contrast with
188 C<$#->, the last I<matched> subgroup. Compare with C<@->.
190 =item $MULTILINE_MATCHING
194 Set to 1 to do multi-line matching within a string, 0 to tell Perl
195 that it can assume that strings contain a single line, for the purpose
196 of optimizing pattern matches. Pattern matches on strings containing
197 multiple newlines can produce confusing results when C<$*> is 0. Default
198 is 0. (Mnemonic: * matches multiple things.) This variable
199 influences the interpretation of only C<^> and C<$>. A literal newline can
200 be searched for even when C<$* == 0>.
202 Use of C<$*> is deprecated in modern Perl, supplanted by
203 the C</s> and C</m> modifiers on pattern matching.
205 =item input_line_number HANDLE EXPR
207 =item $INPUT_LINE_NUMBER
213 The current input record number for the last file handle from which
214 you just read() (or called a C<seek> or C<tell> on). The value
215 may be different from the actual physical line number in the file,
216 depending on what notion of "line" is in effect--see C<$/> on how
217 to change that. An explicit close on a filehandle resets the line
218 number. Because C<E<lt>E<gt>> never does an explicit close, line
219 numbers increase across ARGV files (but see examples in L<perlfunc/eof>).
220 Consider this variable read-only: setting it does not reposition
221 the seek pointer; you'll have to do that on your own. Localizing C<$.>
222 has the effect of also localizing Perl's notion of "the last read
223 filehandle". (Mnemonic: many programs use "." to mean the current line
226 =item input_record_separator HANDLE EXPR
228 =item $INPUT_RECORD_SEPARATOR
234 The input record separator, newline by default. This
235 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
236 variable, including treating empty lines as a terminator if set to
237 the null string. (An empty line cannot contain any spaces
238 or tabs.) You may set it to a multi-character string to match a
239 multi-character terminator, or to C<undef> to read through the end
240 of file. Setting it to C<"\n\n"> means something slightly
241 different than setting to C<"">, if the file contains consecutive
242 empty lines. Setting to C<""> will treat two or more consecutive
243 empty lines as a single empty line. Setting to C<"\n\n"> will
244 blindly assume that the next input character belongs to the next
245 paragraph, even if it's a newline. (Mnemonic: / delimits
246 line boundaries when quoting poetry.)
248 undef $/; # enable "slurp" mode
249 $_ = <FH>; # whole file now here
252 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
253 better for something. :-)
255 Setting C<$/> to a reference to an integer, scalar containing an integer, or
256 scalar that's convertible to an integer will attempt to read records
257 instead of lines, with the maximum record size being the referenced
260 $/ = \32768; # or \"32768", or \$var_containing_32768
264 will read a record of no more than 32768 bytes from FILE. If you're
265 not reading from a record-oriented file (or your OS doesn't have
266 record-oriented files), then you'll likely get a full chunk of data
267 with every read. If a record is larger than the record size you've
268 set, you'll get the record back in pieces.
270 On VMS, record reads are done with the equivalent of C<sysread>,
271 so it's best not to mix record and non-record reads on the same
272 file. (This is unlikely to be a problem, because any file you'd
273 want to read in record mode is probably unusable in line mode.)
274 Non-VMS systems do normal I/O, so it's safe to mix record and
275 non-record reads of a file.
277 See also L<perlport/"Newlines">. Also see C<$.>.
279 =item autoflush HANDLE EXPR
281 =item $OUTPUT_AUTOFLUSH
285 If set to nonzero, forces a flush right away and after every write
286 or print on the currently selected output channel. Default is 0
287 (regardless of whether the channel is really buffered by the
288 system or not; C<$|> tells you only whether you've asked Perl
289 explicitly to flush after each write). STDOUT will
290 typically be line buffered if output is to the terminal and block
291 buffered otherwise. Setting this variable is useful primarily when
292 you are outputting to a pipe or socket, such as when you are running
293 a Perl program under B<rsh> and want to see the output as it's
294 happening. This has no effect on input buffering. See L<perlfunc/getc>
295 for that. (Mnemonic: when you want your pipes to be piping hot.)
297 =item output_field_separator HANDLE EXPR
299 =item $OUTPUT_FIELD_SEPARATOR
305 The output field separator for the print operator. Ordinarily the
306 print operator simply prints out its arguments without further
307 adornment. To get behavior more like B<awk>, set this variable as
308 you would set B<awk>'s OFS variable to specify what is printed
309 between fields. (Mnemonic: what is printed when there is a "," in
310 your print statement.)
312 =item output_record_separator HANDLE EXPR
314 =item $OUTPUT_RECORD_SEPARATOR
320 The output record separator for the print operator. Ordinarily the
321 print operator simply prints out its arguments as is, with no
322 trailing newline or other end-of-record string added. To get
323 behavior more like B<awk>, set this variable as you would set
324 B<awk>'s ORS variable to specify what is printed at the end of the
325 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
326 end of the print. Also, it's just like C<$/>, but it's what you
327 get "back" from Perl.)
329 =item $LIST_SEPARATOR
333 This is like C<$,> except that it applies to array and slice values
334 interpolated into a double-quoted string (or similar interpreted
335 string). Default is a space. (Mnemonic: obvious, I think.)
337 =item $SUBSCRIPT_SEPARATOR
343 The subscript separator for multidimensional array emulation. If you
344 refer to a hash element as
350 $foo{join($;, $a, $b, $c)}
354 @foo{$a,$b,$c} # a slice--note the @
358 ($foo{$a},$foo{$b},$foo{$c})
360 Default is "\034", the same as SUBSEP in B<awk>. If your
361 keys contain binary data there might not be any safe value for C<$;>.
362 (Mnemonic: comma (the syntactic subscript separator) is a
363 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
364 taken for something more important.)
366 Consider using "real" multidimensional arrays as described
373 The output format for printed numbers. This variable is a half-hearted
374 attempt to emulate B<awk>'s OFMT variable. There are times, however,
375 when B<awk> and Perl have differing notions of what counts as
376 numeric. The initial value is "%.I<n>g", where I<n> is the value
377 of the macro DBL_DIG from your system's F<float.h>. This is different from
378 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
379 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
381 Use of C<$#> is deprecated.
383 =item format_page_number HANDLE EXPR
385 =item $FORMAT_PAGE_NUMBER
389 The current page number of the currently selected output channel.
391 (Mnemonic: % is page number in B<nroff>.)
393 =item format_lines_per_page HANDLE EXPR
395 =item $FORMAT_LINES_PER_PAGE
399 The current page length (printable lines) of the currently selected
400 output channel. Default is 60.
402 (Mnemonic: = has horizontal lines.)
404 =item format_lines_left HANDLE EXPR
406 =item $FORMAT_LINES_LEFT
410 The number of lines left on the page of the currently selected output
413 (Mnemonic: lines_on_page - lines_printed.)
417 $-[0] is the offset of the start of the last successful match.
418 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
419 I<n>-th subpattern, or undef if the subpattern did not match.
421 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
422 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
423 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
424 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
425 matched subgroup in the last successful match. Contrast with
426 C<$#+>, the number of subgroups in the regular expression. Compare
429 =item format_name HANDLE EXPR
435 The name of the current report format for the currently selected output
436 channel. Default is the name of the filehandle. (Mnemonic: brother to
439 =item format_top_name HANDLE EXPR
441 =item $FORMAT_TOP_NAME
445 The name of the current top-of-page format for the currently selected
446 output channel. Default is the name of the filehandle with _TOP
447 appended. (Mnemonic: points to top of page.)
449 =item format_line_break_characters HANDLE EXPR
451 =item $FORMAT_LINE_BREAK_CHARACTERS
455 The current set of characters after which a string may be broken to
456 fill continuation fields (starting with ^) in a format. Default is
457 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
458 poetry is a part of a line.)
460 =item format_formfeed HANDLE EXPR
462 =item $FORMAT_FORMFEED
466 What formats output as a form feed. Default is \f.
472 The current value of the write() accumulator for format() lines. A format
473 contains formline() calls that put their result into C<$^A>. After
474 calling its format, write() prints out the contents of C<$^A> and empties.
475 So you never really see the contents of C<$^A> unless you call
476 formline() yourself and then look at it. See L<perlform> and
477 L<perlfunc/formline()>.
483 The status returned by the last pipe close, backtick (C<``>) command,
484 successful call to wait() or waitpid(), or from the system()
485 operator. This is just the 16-bit status word returned by the
486 wait() system call (or else is made up to look like it). Thus, the
487 exit value of the subprocess is really (C<$? E<gt>E<gt> 8>), and
488 C<$? & 127> gives which signal, if any, the process died from, and
489 C<$? & 128> reports whether there was a core dump. (Mnemonic:
490 similar to B<sh> and B<ksh>.)
492 Additionally, if the C<h_errno> variable is supported in C, its value
493 is returned via $? if any C<gethost*()> function fails.
495 If you have installed a signal handler for C<SIGCHLD>, the
496 value of C<$?> will usually be wrong outside that handler.
498 Inside an C<END> subroutine C<$?> contains the value that is going to be
499 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
500 change the exit status of your program. For example:
503 $? = 1 if $? == 255; # die would make it 255
506 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
507 actual VMS exit status, instead of the default emulation of POSIX
510 Also see L<Error Indicators>.
518 If used numerically, yields the current value of the C C<errno>
519 variable, with all the usual caveats. (This means that you shouldn't
520 depend on the value of C<$!> to be anything in particular unless
521 you've gotten a specific error return indicating a system error.)
522 If used an a string, yields the corresponding system error string.
523 You can assign a number to C<$!> to set I<errno> if, for instance,
524 you want C<"$!"> to return the string for error I<n>, or you want
525 to set the exit value for the die() operator. (Mnemonic: What just
528 Also see L<Error Indicators>.
530 =item $EXTENDED_OS_ERROR
534 Error information specific to the current operating system. At
535 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
536 (and for MacPerl). On all other platforms, C<$^E> is always just
539 Under VMS, C<$^E> provides the VMS status value from the last
540 system error. This is more specific information about the last
541 system error than that provided by C<$!>. This is particularly
542 important when C<$!> is set to B<EVMSERR>.
544 Under OS/2, C<$^E> is set to the error code of the last call to
545 OS/2 API either via CRT, or directly from perl.
547 Under Win32, C<$^E> always returns the last error information
548 reported by the Win32 call C<GetLastError()> which describes
549 the last error from within the Win32 API. Most Win32-specific
550 code will report errors via C<$^E>. ANSI C and Unix-like calls
551 set C<errno> and so most portable Perl code will report errors
554 Caveats mentioned in the description of C<$!> generally apply to
555 C<$^E>, also. (Mnemonic: Extra error explanation.)
557 Also see L<Error Indicators>.
563 The Perl syntax error message from the last eval() operator. If null, the
564 last eval() parsed and executed correctly (although the operations you
565 invoked may have failed in the normal fashion). (Mnemonic: Where was
566 the syntax error "at"?)
568 Warning messages are not collected in this variable. You can,
569 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
572 Also see L<Error Indicators>.
580 The process number of the Perl running this script. You should
581 consider this variable read-only, although it will be altered
582 across fork() calls. (Mnemonic: same as shells.)
590 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
591 if you're running setuid.)
593 =item $EFFECTIVE_USER_ID
599 The effective uid of this process. Example:
601 $< = $>; # set real to effective uid
602 ($<,$>) = ($>,$<); # swap real and effective uid
604 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
605 C<$E<lt>> and C<$E<gt>> can be swapped only on machines
606 supporting setreuid().
614 The real gid of this process. If you are on a machine that supports
615 membership in multiple groups simultaneously, gives a space separated
616 list of groups you are in. The first number is the one returned by
617 getgid(), and the subsequent ones by getgroups(), one of which may be
618 the same as the first number.
620 However, a value assigned to C<$(> must be a single number used to
621 set the real gid. So the value given by C<$(> should I<not> be assigned
622 back to C<$(> without being forced numeric, such as by adding zero.
624 (Mnemonic: parentheses are used to I<group> things. The real gid is the
625 group you I<left>, if you're running setgid.)
627 =item $EFFECTIVE_GROUP_ID
633 The effective gid of this process. If you are on a machine that
634 supports membership in multiple groups simultaneously, gives a space
635 separated list of groups you are in. The first number is the one
636 returned by getegid(), and the subsequent ones by getgroups(), one of
637 which may be the same as the first number.
639 Similarly, a value assigned to C<$)> must also be a space-separated
640 list of numbers. The first number sets the effective gid, and
641 the rest (if any) are passed to setgroups(). To get the effect of an
642 empty list for setgroups(), just repeat the new effective gid; that is,
643 to force an effective gid of 5 and an effectively empty setgroups()
644 list, say C< $) = "5 5" >.
646 (Mnemonic: parentheses are used to I<group> things. The effective gid
647 is the group that's I<right> for you, if you're running setgid.)
649 C<$E<lt>>, C<$E<gt>>, C<$(> and C<$)> can be set only on
650 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
651 and C<$)> can be swapped only on machines supporting setregid().
657 Contains the name of the program being executed. On some operating
658 systems assigning to C<$0> modifies the argument area that the B<ps>
659 program sees. This is more useful as a way of indicating the current
660 program state than it is for hiding the program you're running.
661 (Mnemonic: same as B<sh> and B<ksh>.)
665 The index of the first element in an array, and of the first character
666 in a substring. Default is 0, but you could theoretically set it
667 to 1 to make Perl behave more like B<awk> (or Fortran) when
668 subscripting and when evaluating the index() and substr() functions.
669 (Mnemonic: [ begins subscripts.)
671 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
672 directive, and cannot influence the behavior of any other file.
673 Its use is highly discouraged.
679 The version + patchlevel / 1000 of the Perl interpreter. This variable
680 can be used to determine whether the Perl interpreter executing a
681 script is in the right range of versions. (Mnemonic: Is this version
682 of perl in the right bracket?) Example:
684 warn "No checksumming!\n" if $] < 3.019;
686 See also the documentation of C<use VERSION> and C<require VERSION>
687 for a convenient way to fail if the running Perl interpreter is too old.
689 See C<$^V> for a more modern representation of the Perl version.
695 The current value of the flag associated with the B<-c> switch.
696 Mainly of use with B<-MO=...> to allow code to alter its behavior
697 when being compiled, such as for example to AUTOLOAD at compile
698 time rather than normal, deferred loading. See L<perlcc>. Setting
699 C<$^C = 1> is similar to calling C<B::minus_c>.
705 The current value of the debugging flags. (Mnemonic: value of B<-D>
712 The maximum system file descriptor, ordinarily 2. System file
713 descriptors are passed to exec()ed processes, while higher file
714 descriptors are not. Also, during an open(), system file descriptors are
715 preserved even if the open() fails. (Ordinary file descriptors are
716 closed before the open() is attempted.) The close-on-exec
717 status of a file descriptor will be decided according to the value of
718 C<$^F> when the open() or pipe() was called, not the time of the exec().
722 WARNING: This variable is strictly for internal use only. Its availability,
723 behavior, and contents are subject to change without notice.
725 This variable contains compile-time hints for the Perl interpreter. At the
726 end of compilation of a BLOCK the value of this variable is restored to the
727 value when the interpreter started to compile the BLOCK.
729 When perl begins to parse any block construct that provides a lexical scope
730 (e.g., eval body, required file, subroutine body, loop body, or conditional
731 block), the existing value of $^H is saved, but its value is left unchanged.
732 When the compilation of the block is completed, it regains the saved value.
733 Between the points where its value is saved and restored, code that
734 executes within BEGIN blocks is free to change the value of $^H.
736 This behavior provides the semantic of lexical scoping, and is used in,
737 for instance, the C<use strict> pragma.
739 The contents should be an integer; different bits of it are used for
740 different pragmatic flags. Here's an example:
742 sub add_100 { $^H |= 0x100 }
749 Consider what happens during execution of the BEGIN block. At this point
750 the BEGIN block has already been compiled, but the body of foo() is still
751 being compiled. The new value of $^H will therefore be visible only while
752 the body of foo() is being compiled.
754 Substitution of the above BEGIN block with:
756 BEGIN { require strict; strict->import('vars') }
758 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
759 version of the same lexical pragma:
761 BEGIN { require strict; strict->import('vars') if $condition }
765 WARNING: This variable is strictly for internal use only. Its availability,
766 behavior, and contents are subject to change without notice.
768 The %^H hash provides the same scoping semantic as $^H. This makes it
769 useful for implementation of lexically scoped pragmas.
775 The current value of the inplace-edit extension. Use C<undef> to disable
776 inplace editing. (Mnemonic: value of B<-i> switch.)
780 By default, running out of memory is an untrappable, fatal error.
781 However, if suitably built, Perl can use the contents of C<$^M>
782 as an emergency memory pool after die()ing. Suppose that your Perl
783 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
786 $^M = 'a' x (1 << 16);
788 would allocate a 64K buffer for use when in emergency. See the
789 F<INSTALL> file in the Perl distribution for information on how to
790 enable this option. To discourage casual use of this advanced
791 feature, there is no L<English> long name for this variable.
797 The name of the operating system under which this copy of Perl was
798 built, as determined during the configuration process. The value
799 is identical to C<$Config{'osname'}>. See also L<Config> and the
800 B<-V> command-line switch documented in L<perlrun>.
806 The internal variable for debugging support. The meanings of the
807 various bits are subject to change, but currently indicate:
813 Debug subroutine enter/exit.
817 Line-by-line debugging.
821 Switch off optimizations.
825 Preserve more data for future interactive inspections.
829 Keep info about source lines on which a subroutine is defined.
833 Start with single-step on.
837 Some bits may be relevant at compile-time only, some at
838 run-time only. This is a new mechanism and the details may change.
842 The result of evaluation of the last successful C<(?{ code })>
843 regular expression assertion (see L<perlre>). May be written to.
847 Current state of the interpreter. Undefined if parsing of the current
848 module/eval is not finished (may happen in $SIG{__DIE__} and
849 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
855 The time at which the program began running, in seconds since the
856 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
857 and B<-C> filetests are based on this value.
861 Global flag that switches on Unicode character support in the Perl
862 interpreter. The initial value is usually C<0> for compatibility
863 with Perl versions earlier than 5.6, but may be automatically set
864 to C<1> by Perl if the system provides a user-settable default
865 (e.g., C<$ENV{LC_CTYPE}>). It is also implicitly set to C<1>
866 whenever the utf8 pragma is loaded.
868 Setting it to C<1> has the following effects:
874 C<chr> produces UTF-8 encoded Unicode characters. These are the same
875 as the corresponding ASCII characters if the argument is less than 128.
879 The C<%c> format in C<sprintf> generates a UTF-8 encoded Unicode
880 character. This is the same as the corresponding ASCII character
881 if the argument is less than 128.
885 Any system calls made by Perl will use wide character APIs native to
886 the system, if available. This is currently only implemented on the
891 The C<byte> pragma overrides the value of this flag in the current
892 lexical scope. See L<byte>.
896 The revision, version, and subversion of the Perl interpreter, represented
897 as a "version tuple". Version tuples have both a numeric value and a
898 string value. The numeric value is a floating point number that amounts
899 to revision + version/1000 + subversion/1000000, and the string value
900 is made of utf8 characters:
901 C<chr($revision) . chr($version) . chr($subversion)>.
903 This can be used to determine whether the Perl interpreter executing a
904 script is in the right range of versions. (Mnemonic: use ^V for Version
907 warn "No "our" declarations!\n" if $^V and $^V lt v5.6;
909 See also the documentation of C<use VERSION> and C<require VERSION>
910 for a convenient way to fail if the running Perl interpreter is too old.
912 See also C<$]> for an older representation of the Perl version.
918 The current value of the warning switch, initially true if B<-w>
919 was used, false otherwise, but directly modifiable. (Mnemonic:
920 related to the B<-w> switch.) See also L<warnings>.
924 The current set of warning checks enabled by the C<use warnings> pragma.
925 See the documentation of C<warnings> for more details.
927 =item $EXECUTABLE_NAME
931 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
932 This may not be a full pathname, nor even necessarily in your path.
936 contains the name of the current file when reading from E<lt>E<gt>.
940 The array @ARGV contains the command-line arguments intended for
941 the script. C<$#ARGV> is generally the number of arguments minus
942 one, because C<$ARGV[0]> is the first argument, I<not> the program's
943 command name itself. See C<$0> for the command name.
947 The array @INC contains the list of places that the C<do EXPR>,
948 C<require>, or C<use> constructs look for their library files. It
949 initially consists of the arguments to any B<-I> command-line
950 switches, followed by the default Perl library, probably
951 F</usr/local/lib/perl>, followed by ".", to represent the current
952 directory. If you need to modify this at runtime, you should use
953 the C<use lib> pragma to get the machine-dependent library properly
956 use lib '/mypath/libdir/';
961 Within a subroutine the array @_ contains the parameters passed to that
962 subroutine. See L<perlsub>.
966 The hash %INC contains entries for each filename included via the
967 C<do>, C<require>, or C<use> operators. The key is the filename
968 you specified (with module names converted to pathnames), and the
969 value is the location of the file found. The C<require>
970 operator uses this hash to determine whether a particular file has
971 already been included.
977 The hash %ENV contains your current environment. Setting a
978 value in C<ENV> changes the environment for any child processes
979 you subsequently fork() off.
985 The hash %SIG contains signal handlers for signals. For example:
987 sub handler { # 1st argument is signal name
989 print "Caught a SIG$sig--shutting down\n";
994 $SIG{'INT'} = \&handler;
995 $SIG{'QUIT'} = \&handler;
997 $SIG{'INT'} = 'DEFAULT'; # restore default action
998 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1000 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1001 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1004 Here are some other examples:
1006 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1007 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1008 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1009 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1011 Be sure not to use a bareword as the name of a signal handler,
1012 lest you inadvertently call it.
1014 If your system has the sigaction() function then signal handlers are
1015 installed using it. This means you get reliable signal handling. If
1016 your system has the SA_RESTART flag it is used when signals handlers are
1017 installed. This means that system calls for which restarting is supported
1018 continue rather than returning when a signal arrives. If you want your
1019 system calls to be interrupted by signal delivery then do something like
1022 use POSIX ':signal_h';
1025 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1026 or die "Error setting SIGALRM handler: $!\n";
1030 Certain internal hooks can be also set using the %SIG hash. The
1031 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1032 about to be printed. The warning message is passed as the first
1033 argument. The presence of a __WARN__ hook causes the ordinary printing
1034 of warnings to STDERR to be suppressed. You can use this to save warnings
1035 in a variable, or turn warnings into fatal errors, like this:
1037 local $SIG{__WARN__} = sub { die $_[0] };
1040 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1041 is about to be thrown. The error message is passed as the first
1042 argument. When a __DIE__ hook routine returns, the exception
1043 processing continues as it would have in the absence of the hook,
1044 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1045 The C<__DIE__> handler is explicitly disabled during the call, so that you
1046 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1048 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1049 even inside an eval(). Do not use this to rewrite a pending exception
1050 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1051 This strange action at a distance may be fixed in a future release
1052 so that C<$SIG{__DIE__}> is only called if your program is about
1053 to exit, as was the original intent. Any other use is deprecated.
1055 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1056 they may be called to report (probable) errors found by the parser.
1057 In such a case the parser may be in inconsistent state, so any
1058 attempt to evaluate Perl code from such a handler will probably
1059 result in a segfault. This means that warnings or errors that
1060 result from parsing Perl should be used with extreme caution, like
1063 require Carp if defined $^S;
1064 Carp::confess("Something wrong") if defined &Carp::confess;
1065 die "Something wrong, but could not load Carp to give backtrace...
1066 To see backtrace try starting Perl with -MCarp switch";
1068 Here the first line will load Carp I<unless> it is the parser who
1069 called the handler. The second line will print backtrace and die if
1070 Carp was available. The third line will be executed only if Carp was
1073 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1074 L<warnings> for additional information.
1078 =head2 Error Indicators
1080 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1081 about different types of error conditions that may appear during
1082 execution of a Perl program. The variables are shown ordered by
1083 the "distance" between the subsystem which reported the error and
1084 the Perl process. They correspond to errors detected by the Perl
1085 interpreter, C library, operating system, or an external program,
1088 To illustrate the differences between these variables, consider the
1089 following Perl expression, which uses a single-quoted string:
1092 open PIPE, "/cdrom/install |";
1094 close PIPE or die "bad pipe: $?, $!";
1097 After execution of this statement all 4 variables may have been set.
1099 C<$@> is set if the string to be C<eval>-ed did not compile (this
1100 may happen if C<open> or C<close> were imported with bad prototypes),
1101 or if Perl code executed during evaluation die()d . In these cases
1102 the value of $@ is the compile error, or the argument to C<die>
1103 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1106 When the eval() expression above is executed, open(), C<<PIPEE<gt>>,
1107 and C<close> are translated to calls in the C run-time library and
1108 thence to the operating system kernel. C<$!> is set to the C library's
1109 C<errno> if one of these calls fails.
1111 Under a few operating systems, C<$^E> may contain a more verbose
1112 error indicator, such as in this case, "CDROM tray not closed."
1113 Systems that do not support extended error messages leave C<$^E>
1116 Finally, C<$?> may be set to non-0 value if the external program
1117 F</cdrom/install> fails. The upper eight bits reflect specific
1118 error conditions encountered by the program (the program's exit()
1119 value). The lower eight bits reflect mode of failure, like signal
1120 death and core dump information See wait(2) for details. In
1121 contrast to C<$!> and C<$^E>, which are set only if error condition
1122 is detected, the variable C<$?> is set on each C<wait> or pipe
1123 C<close>, overwriting the old value. This is more like C<$@>, which
1124 on every eval() is always set on failure and cleared on success.
1126 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1129 =head2 Technical Note on the Syntax of Variable Names
1131 Variable names in Perl can have several formats. Usually, they
1132 must begin with a letter or underscore, in which case they can be
1133 arbitrarily long (up to an internal limit of 251 characters) and
1134 may contain letters, digits, underscores, or the special sequence
1135 C<::> or C<'>. In this case, the part before the last C<::> or
1136 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1138 Perl variable names may also be a sequence of digits or a single
1139 punctuation or control character. These names are all reserved for
1140 special uses by Perl; for example, the all-digits names are used
1141 to hold data captured by backreferences after a regular expression
1142 match. Perl has a special syntax for the single-control-character
1143 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1144 character. For example, the notation C<$^W> (dollar-sign caret
1145 C<W>) is the scalar variable whose name is the single character
1146 control-C<W>. This is better than typing a literal control-C<W>
1149 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1150 strings that begin with control characters (or better yet, a caret).
1151 These variables must be written in the form C<${^Foo}>; the braces
1152 are not optional. C<${^Foo}> denotes the scalar variable whose
1153 name is a control-C<F> followed by two C<o>'s. These variables are
1154 reserved for future special uses by Perl, except for the ones that
1155 begin with C<^_> (control-underscore or caret-underscore). No
1156 control-character name that begins with C<^_> will acquire a special
1157 meaning in any future version of Perl; such names may therefore be
1158 used safely in programs. C<$^_> itself, however, I<is> reserved.
1160 Perl identifiers that begin with digits, control characters, or
1161 punctuation characters are exempt from the effects of the C<package>
1162 declaration and are always forced to be in package C<main>. A few
1163 other names are also exempt:
1171 In particular, the new special C<${^_XYZ}> variables are always taken
1172 to be in package C<main>, regardless of any C<package> declarations
1177 Due to an unfortunate accident of Perl's implementation, C<use
1178 English> imposes a considerable performance penalty on all regular
1179 expression matches in a program, regardless of whether they occur
1180 in the scope of C<use English>. For that reason, saying C<use
1181 English> in libraries is strongly discouraged. See the
1182 Devel::SawAmpersand module documentation from CPAN
1183 (http://www.perl.com/CPAN/modules/by-module/Devel/Devel-SawAmpersand-0.10.readme)
1184 for more information.
1186 Having to even think about the C<$^S> variable in your exception
1187 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1188 invites grievous and difficult to track down errors. Avoid it
1189 and use an C<END{}> or CORE::GLOBAL::die override instead.