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 You should be very careful when modifying the default values of most
48 special variables described in this document. In most cases you want
49 to localize these variables before changing them, since if you don't,
50 the change may affect other modules which rely on the default values
51 of the special variables that you have changed. This is one of the
52 correct ways to read the whole file at once:
54 open my $fh, "foo" or die $!;
55 local $/; # enable localized slurp mode
59 But the following code is quite bad:
61 open my $fh, "foo" or die $!;
62 undef $/; # enable slurp mode
66 since some other module, may want to read data from some file in the
67 default "line mode", so if the code we have just presented has been
68 executed, the global value of C<$/> is now changed for any other code
69 running inside the same Perl interpreter.
71 Usually when a variable is localized you want to make sure that this
72 change affects the shortest scope possible. So unless you are already
73 inside some short C<{}> block, you should create one yourself. For
77 open my $fh, "foo" or die $!;
84 Here is an example of how your own code can go broken:
92 # do something with $_
95 You probably expect this code to print:
103 Why? Because nasty_break() modifies C<$_> without localizing it
104 first. The fix is to add local():
108 It's easy to notice the problem in such a short example, but in more
109 complicated code you are looking for trouble if you don't localize
110 changes to the special variables.
112 The following list is ordered by scalar variables first, then the
113 arrays, then the hashes.
121 The default input and pattern-searching space. The following pairs are
124 while (<>) {...} # equivalent only in while!
125 while (defined($_ = <>)) {...}
136 Here are the places where Perl will assume $_ even if you
143 Various unary functions, including functions like ord() and int(), as well
144 as the all file tests (C<-f>, C<-d>) except for C<-t>, which defaults to
149 Various list functions like print() and unlink().
153 The pattern matching operations C<m//>, C<s///>, and C<tr///> when used
154 without an C<=~> operator.
158 The default iterator variable in a C<foreach> loop if no other
159 variable is supplied.
163 The implicit iterator variable in the grep() and map() functions.
167 The default place to put an input record when a C<< <FH> >>
168 operation's result is tested by itself as the sole criterion of a C<while>
169 test. Outside a C<while> test, this will not happen.
173 (Mnemonic: underline is understood in certain operations.)
183 Special package variables when using sort(), see L<perlfunc/sort>.
184 Because of this specialness $a and $b don't need to be declared
185 (using local(), use vars, or our()) even when using the strict
186 vars pragma. Don't lexicalize them with C<my $a> or C<my $b>
187 if you want to be able to use them in the sort() comparison block
196 Contains the subpattern from the corresponding set of capturing
197 parentheses from the last pattern match, not counting patterns
198 matched in nested blocks that have been exited already. (Mnemonic:
199 like \digits.) These variables are all read-only and dynamically
200 scoped to the current BLOCK.
206 The string matched by the last successful pattern match (not counting
207 any matches hidden within a BLOCK or eval() enclosed by the current
208 BLOCK). (Mnemonic: like & in some editors.) This variable is read-only
209 and dynamically scoped to the current BLOCK.
211 The use of this variable anywhere in a program imposes a considerable
212 performance penalty on all regular expression matches. See L<BUGS>.
218 The string preceding whatever was matched by the last successful
219 pattern match (not counting any matches hidden within a BLOCK or eval
220 enclosed by the current BLOCK). (Mnemonic: C<`> often precedes a quoted
221 string.) This variable is read-only.
223 The use of this variable anywhere in a program imposes a considerable
224 performance penalty on all regular expression matches. See L<BUGS>.
230 The string following whatever was matched by the last successful
231 pattern match (not counting any matches hidden within a BLOCK or eval()
232 enclosed by the current BLOCK). (Mnemonic: C<'> often follows a quoted
235 local $_ = 'abcdefghi';
237 print "$`:$&:$'\n"; # prints abc:def:ghi
239 This variable is read-only and dynamically scoped to the current BLOCK.
241 The use of this variable anywhere in a program imposes a considerable
242 performance penalty on all regular expression matches. See L<BUGS>.
244 =item $LAST_PAREN_MATCH
248 The text matched by the last bracket of the last successful search pattern.
249 This is useful if you don't know which one of a set of alternative patterns
250 matched. For example:
252 /Version: (.*)|Revision: (.*)/ && ($rev = $+);
254 (Mnemonic: be positive and forward looking.)
255 This variable is read-only and dynamically scoped to the current BLOCK.
259 The text matched by the used group most-recently closed (i.e. the group
260 with the rightmost closing parenthesis) of the last successful search
261 pattern. (Mnemonic: the (possibly) Nested parenthesis that most
264 This is primarly used inside C<(?{...})> blocks for examining text
265 recently matched. For example, to effectively capture text to a variable
266 (in addition to C<$1>, C<$2>, etc.), replace C<(...)> with
268 (?:(...)(?{ $var = $^N }))
270 By setting and then using C<$var> in this way relieves you from having to
271 worry about exactly which numbered set of parentheses they are.
273 This variable is dynamically scoped to the current BLOCK.
275 =item @LAST_MATCH_END
279 This array holds the offsets of the ends of the last successful
280 submatches in the currently active dynamic scope. C<$+[0]> is
281 the offset into the string of the end of the entire match. This
282 is the same value as what the C<pos> function returns when called
283 on the variable that was matched against. The I<n>th element
284 of this array holds the offset of the I<n>th submatch, so
285 C<$+[1]> is the offset past where $1 ends, C<$+[2]> the offset
286 past where $2 ends, and so on. You can use C<$#+> to determine
287 how many subgroups were in the last successful match. See the
288 examples given for the C<@-> variable.
290 =item $MULTILINE_MATCHING
294 Set to a non-zero integer value to do multi-line matching within a
295 string, 0 (or undefined) to tell Perl that it can assume that strings
296 contain a single line, for the purpose of optimizing pattern matches.
297 Pattern matches on strings containing multiple newlines can produce
298 confusing results when C<$*> is 0 or undefined. Default is undefined.
299 (Mnemonic: * matches multiple things.) This variable influences the
300 interpretation of only C<^> and C<$>. A literal newline can be searched
301 for even when C<$* == 0>.
303 Use of C<$*> is deprecated in modern Perl, supplanted by
304 the C</s> and C</m> modifiers on pattern matching.
306 Assigning a non-numerical value to C<$*> triggers a warning (and makes
307 C<$*> act if C<$* == 0>), while assigning a numerical value to C<$*>
308 makes that an implicit C<int> is applied on the value.
310 =item HANDLE->input_line_number(EXPR)
312 =item $INPUT_LINE_NUMBER
318 Current line number for the last filehandle accessed.
320 Each filehandle in Perl counts the number of lines that have been read
321 from it. (Depending on the value of C<$/>, Perl's idea of what
322 constitutes a line may not match yours.) When a line is read from a
323 filehandle (via readline() or C<< <> >>), or when tell() or seek() is
324 called on it, C<$.> becomes an alias to the line counter for that
327 You can adjust the counter by assigning to C<$.>, but this will not
328 actually move the seek pointer. I<Localizing C<$.> will not localize
329 the filehandle's line count>. Instead, it will localize perl's notion
330 of which filehandle C<$.> is currently aliased to.
332 C<$.> is reset when the filehandle is closed, but B<not> when an open
333 filehandle is reopened without an intervening close(). For more
334 details, see L<perlop/"I/O Operators">. Because C<< <> >> never does
335 an explicit close, line numbers increase across ARGV files (but see
336 examples in L<perlfunc/eof>).
338 You can also use C<< HANDLE->input_line_number(EXPR) >> to access the
339 line counter for a given filehandle without having to worry about
340 which handle you last accessed.
342 (Mnemonic: many programs use "." to mean the current line number.)
344 =item IO::Handle->input_record_separator(EXPR)
346 =item $INPUT_RECORD_SEPARATOR
352 The input record separator, newline by default. This
353 influences Perl's idea of what a "line" is. Works like B<awk>'s RS
354 variable, including treating empty lines as a terminator if set to
355 the null string. (An empty line cannot contain any spaces
356 or tabs.) You may set it to a multi-character string to match a
357 multi-character terminator, or to C<undef> to read through the end
358 of file. Setting it to C<"\n\n"> means something slightly
359 different than setting to C<"">, if the file contains consecutive
360 empty lines. Setting to C<""> will treat two or more consecutive
361 empty lines as a single empty line. Setting to C<"\n\n"> will
362 blindly assume that the next input character belongs to the next
363 paragraph, even if it's a newline. (Mnemonic: / delimits
364 line boundaries when quoting poetry.)
366 local $/; # enable "slurp" mode
367 local $_ = <FH>; # whole file now here
370 Remember: the value of C<$/> is a string, not a regex. B<awk> has to be
371 better for something. :-)
373 Setting C<$/> to a reference to an integer, scalar containing an integer, or
374 scalar that's convertible to an integer will attempt to read records
375 instead of lines, with the maximum record size being the referenced
378 local $/ = \32768; # or \"32768", or \$var_containing_32768
379 open my $fh, $myfile or die $!;
382 will read a record of no more than 32768 bytes from FILE. If you're
383 not reading from a record-oriented file (or your OS doesn't have
384 record-oriented files), then you'll likely get a full chunk of data
385 with every read. If a record is larger than the record size you've
386 set, you'll get the record back in pieces.
388 On VMS, record reads are done with the equivalent of C<sysread>,
389 so it's best not to mix record and non-record reads on the same
390 file. (This is unlikely to be a problem, because any file you'd
391 want to read in record mode is probably unusable in line mode.)
392 Non-VMS systems do normal I/O, so it's safe to mix record and
393 non-record reads of a file.
395 See also L<perlport/"Newlines">. Also see C<$.>.
397 =item HANDLE->autoflush(EXPR)
399 =item $OUTPUT_AUTOFLUSH
403 If set to nonzero, forces a flush right away and after every write
404 or print on the currently selected output channel. Default is 0
405 (regardless of whether the channel is really buffered by the
406 system or not; C<$|> tells you only whether you've asked Perl
407 explicitly to flush after each write). STDOUT will
408 typically be line buffered if output is to the terminal and block
409 buffered otherwise. Setting this variable is useful primarily when
410 you are outputting to a pipe or socket, such as when you are running
411 a Perl program under B<rsh> and want to see the output as it's
412 happening. This has no effect on input buffering. See L<perlfunc/getc>
413 for that. (Mnemonic: when you want your pipes to be piping hot.)
415 =item IO::Handle->output_field_separator EXPR
417 =item $OUTPUT_FIELD_SEPARATOR
423 The output field separator for the print operator. Ordinarily the
424 print operator simply prints out its arguments without further
425 adornment. To get behavior more like B<awk>, set this variable as
426 you would set B<awk>'s OFS variable to specify what is printed
427 between fields. (Mnemonic: what is printed when there is a "," in
428 your print statement.)
430 =item IO::Handle->output_record_separator EXPR
432 =item $OUTPUT_RECORD_SEPARATOR
438 The output record separator for the print operator. Ordinarily the
439 print operator simply prints out its arguments as is, with no
440 trailing newline or other end-of-record string added. To get
441 behavior more like B<awk>, set this variable as you would set
442 B<awk>'s ORS variable to specify what is printed at the end of the
443 print. (Mnemonic: you set C<$\> instead of adding "\n" at the
444 end of the print. Also, it's just like C<$/>, but it's what you
445 get "back" from Perl.)
447 =item $LIST_SEPARATOR
451 This is like C<$,> except that it applies to array and slice values
452 interpolated into a double-quoted string (or similar interpreted
453 string). Default is a space. (Mnemonic: obvious, I think.)
455 =item $SUBSCRIPT_SEPARATOR
461 The subscript separator for multidimensional array emulation. If you
462 refer to a hash element as
468 $foo{join($;, $a, $b, $c)}
472 @foo{$a,$b,$c} # a slice--note the @
476 ($foo{$a},$foo{$b},$foo{$c})
478 Default is "\034", the same as SUBSEP in B<awk>. If your
479 keys contain binary data there might not be any safe value for C<$;>.
480 (Mnemonic: comma (the syntactic subscript separator) is a
481 semi-semicolon. Yeah, I know, it's pretty lame, but C<$,> is already
482 taken for something more important.)
484 Consider using "real" multidimensional arrays as described
491 The output format for printed numbers. This variable is a half-hearted
492 attempt to emulate B<awk>'s OFMT variable. There are times, however,
493 when B<awk> and Perl have differing notions of what counts as
494 numeric. The initial value is "%.I<n>g", where I<n> is the value
495 of the macro DBL_DIG from your system's F<float.h>. This is different from
496 B<awk>'s default OFMT setting of "%.6g", so you need to set C<$#>
497 explicitly to get B<awk>'s value. (Mnemonic: # is the number sign.)
499 Use of C<$#> is deprecated.
501 =item HANDLE->format_page_number(EXPR)
503 =item $FORMAT_PAGE_NUMBER
507 The current page number of the currently selected output channel.
509 (Mnemonic: % is page number in B<nroff>.)
511 =item HANDLE->format_lines_per_page(EXPR)
513 =item $FORMAT_LINES_PER_PAGE
517 The current page length (printable lines) of the currently selected
518 output channel. Default is 60.
520 (Mnemonic: = has horizontal lines.)
522 =item HANDLE->format_lines_left(EXPR)
524 =item $FORMAT_LINES_LEFT
528 The number of lines left on the page of the currently selected output
531 (Mnemonic: lines_on_page - lines_printed.)
533 =item @LAST_MATCH_START
537 $-[0] is the offset of the start of the last successful match.
538 C<$-[>I<n>C<]> is the offset of the start of the substring matched by
539 I<n>-th subpattern, or undef if the subpattern did not match.
541 Thus after a match against $_, $& coincides with C<substr $_, $-[0],
542 $+[0] - $-[0]>. Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
543 $+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
544 C<substr $_, $-[$#-], $+[$#-]>. One can use C<$#-> to find the last
545 matched subgroup in the last successful match. Contrast with
546 C<$#+>, the number of subgroups in the regular expression. Compare
549 This array holds the offsets of the beginnings of the last
550 successful submatches in the currently active dynamic scope.
551 C<$-[0]> is the offset into the string of the beginning of the
552 entire match. The I<n>th element of this array holds the offset
553 of the I<n>th submatch, so C<$-[1]> is the offset where $1
554 begins, C<$-[2]> the offset where $2 begins, and so on.
556 After a match against some variable $var:
560 =item C<$`> is the same as C<substr($var, 0, $-[0])>
562 =item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>
564 =item C<$'> is the same as C<substr($var, $+[0])>
566 =item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>
568 =item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>
570 =item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>
574 =item HANDLE->format_name(EXPR)
580 The name of the current report format for the currently selected output
581 channel. Default is the name of the filehandle. (Mnemonic: brother to
584 =item HANDLE->format_top_name(EXPR)
586 =item $FORMAT_TOP_NAME
590 The name of the current top-of-page format for the currently selected
591 output channel. Default is the name of the filehandle with _TOP
592 appended. (Mnemonic: points to top of page.)
594 =item IO::Handle->format_line_break_characters EXPR
596 =item $FORMAT_LINE_BREAK_CHARACTERS
600 The current set of characters after which a string may be broken to
601 fill continuation fields (starting with ^) in a format. Default is
602 S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in
603 poetry is a part of a line.)
605 =item IO::Handle->format_formfeed EXPR
607 =item $FORMAT_FORMFEED
611 What formats output as a form feed. Default is \f.
617 The current value of the write() accumulator for format() lines. A format
618 contains formline() calls that put their result into C<$^A>. After
619 calling its format, write() prints out the contents of C<$^A> and empties.
620 So you never really see the contents of C<$^A> unless you call
621 formline() yourself and then look at it. See L<perlform> and
622 L<perlfunc/formline()>.
628 The status returned by the last pipe close, backtick (C<``>) command,
629 successful call to wait() or waitpid(), or from the system()
630 operator. This is just the 16-bit status word returned by the
631 wait() system call (or else is made up to look like it). Thus, the
632 exit value of the subprocess is really (C<<< $? >> 8 >>>), and
633 C<$? & 127> gives which signal, if any, the process died from, and
634 C<$? & 128> reports whether there was a core dump. (Mnemonic:
635 similar to B<sh> and B<ksh>.)
637 Additionally, if the C<h_errno> variable is supported in C, its value
638 is returned via $? if any C<gethost*()> function fails.
640 If you have installed a signal handler for C<SIGCHLD>, the
641 value of C<$?> will usually be wrong outside that handler.
643 Inside an C<END> subroutine C<$?> contains the value that is going to be
644 given to C<exit()>. You can modify C<$?> in an C<END> subroutine to
645 change the exit status of your program. For example:
648 $? = 1 if $? == 255; # die would make it 255
651 Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
652 actual VMS exit status, instead of the default emulation of POSIX
653 status; see L<perlvms/$?> for details.
655 Also see L<Error Indicators>.
659 The encoding used to interpret native eight-bit encodings to Unicode,
660 see L<encode>. An opaque C<Encode::XS> object.
668 If used numerically, yields the current value of the C C<errno>
669 variable, with all the usual caveats. (This means that you shouldn't
670 depend on the value of C<$!> to be anything in particular unless
671 you've gotten a specific error return indicating a system error.)
672 If used an a string, yields the corresponding system error string.
673 You can assign a number to C<$!> to set I<errno> if, for instance,
674 you want C<"$!"> to return the string for error I<n>, or you want
675 to set the exit value for the die() operator. (Mnemonic: What just
678 Also see L<Error Indicators>.
680 =item $EXTENDED_OS_ERROR
684 Error information specific to the current operating system. At
685 the moment, this differs from C<$!> under only VMS, OS/2, and Win32
686 (and for MacPerl). On all other platforms, C<$^E> is always just
689 Under VMS, C<$^E> provides the VMS status value from the last
690 system error. This is more specific information about the last
691 system error than that provided by C<$!>. This is particularly
692 important when C<$!> is set to B<EVMSERR>.
694 Under OS/2, C<$^E> is set to the error code of the last call to
695 OS/2 API either via CRT, or directly from perl.
697 Under Win32, C<$^E> always returns the last error information
698 reported by the Win32 call C<GetLastError()> which describes
699 the last error from within the Win32 API. Most Win32-specific
700 code will report errors via C<$^E>. ANSI C and Unix-like calls
701 set C<errno> and so most portable Perl code will report errors
704 Caveats mentioned in the description of C<$!> generally apply to
705 C<$^E>, also. (Mnemonic: Extra error explanation.)
707 Also see L<Error Indicators>.
713 The Perl syntax error message from the last eval() operator.
714 If $@ is the null string, the last eval() parsed and executed
715 correctly (although the operations you invoked may have failed in the
716 normal fashion). (Mnemonic: Where was the syntax error "at"?)
718 Warning messages are not collected in this variable. You can,
719 however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
722 Also see L<Error Indicators>.
730 The process number of the Perl running this script. You should
731 consider this variable read-only, although it will be altered
732 across fork() calls. (Mnemonic: same as shells.)
740 The real uid of this process. (Mnemonic: it's the uid you came I<from>,
741 if you're running setuid.) You can change both the real uid and
742 the effective uid at the same time by using POSIX::setuid().
744 =item $EFFECTIVE_USER_ID
750 The effective uid of this process. Example:
752 $< = $>; # set real to effective uid
753 ($<,$>) = ($>,$<); # swap real and effective uid
755 You can change both the effective uid and the real uid at the same
756 time by using POSIX::setuid().
758 (Mnemonic: it's the uid you went I<to>, if you're running setuid.)
759 C<< $< >> and C<< $> >> can be swapped only on machines
760 supporting setreuid().
768 The real gid of this process. If you are on a machine that supports
769 membership in multiple groups simultaneously, gives a space separated
770 list of groups you are in. The first number is the one returned by
771 getgid(), and the subsequent ones by getgroups(), one of which may be
772 the same as the first number.
774 However, a value assigned to C<$(> must be a single number used to
775 set the real gid. So the value given by C<$(> should I<not> be assigned
776 back to C<$(> without being forced numeric, such as by adding zero.
778 You can change both the real gid and the effective gid at the same
779 time by using POSIX::setgid().
781 (Mnemonic: parentheses are used to I<group> things. The real gid is the
782 group you I<left>, if you're running setgid.)
784 =item $EFFECTIVE_GROUP_ID
790 The effective gid of this process. If you are on a machine that
791 supports membership in multiple groups simultaneously, gives a space
792 separated list of groups you are in. The first number is the one
793 returned by getegid(), and the subsequent ones by getgroups(), one of
794 which may be the same as the first number.
796 Similarly, a value assigned to C<$)> must also be a space-separated
797 list of numbers. The first number sets the effective gid, and
798 the rest (if any) are passed to setgroups(). To get the effect of an
799 empty list for setgroups(), just repeat the new effective gid; that is,
800 to force an effective gid of 5 and an effectively empty setgroups()
801 list, say C< $) = "5 5" >.
803 You can change both the effective gid and the real gid at the same
804 time by using POSIX::setgid() (use only a single numeric argument).
806 (Mnemonic: parentheses are used to I<group> things. The effective gid
807 is the group that's I<right> for you, if you're running setgid.)
809 C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
810 machines that support the corresponding I<set[re][ug]id()> routine. C<$(>
811 and C<$)> can be swapped only on machines supporting setregid().
817 Contains the name of the program being executed. On some operating
818 systems assigning to C<$0> modifies the argument area that the B<ps>
819 program sees. This is more useful as a way of indicating the current
820 program state than it is for hiding the program you're running.
821 (Mnemonic: same as B<sh> and B<ksh>.)
823 Note for BSD users: setting C<$0> does not completely remove "perl"
824 from the ps(1) output. For example, setting C<$0> to C<"foobar"> will
825 result in C<"perl: foobar (perl)">. This is an operating system
830 The index of the first element in an array, and of the first character
831 in a substring. Default is 0, but you could theoretically set it
832 to 1 to make Perl behave more like B<awk> (or Fortran) when
833 subscripting and when evaluating the index() and substr() functions.
834 (Mnemonic: [ begins subscripts.)
836 As of release 5 of Perl, assignment to C<$[> is treated as a compiler
837 directive, and cannot influence the behavior of any other file.
838 Its use is highly discouraged.
842 The version + patchlevel / 1000 of the Perl interpreter. This variable
843 can be used to determine whether the Perl interpreter executing a
844 script is in the right range of versions. (Mnemonic: Is this version
845 of perl in the right bracket?) Example:
847 warn "No checksumming!\n" if $] < 3.019;
849 See also the documentation of C<use VERSION> and C<require VERSION>
850 for a convenient way to fail if the running Perl interpreter is too old.
852 The floating point representation can sometimes lead to inaccurate
853 numeric comparisons. See C<$^V> for a more modern representation of
854 the Perl version that allows accurate string comparisons.
860 The current value of the flag associated with the B<-c> switch.
861 Mainly of use with B<-MO=...> to allow code to alter its behavior
862 when being compiled, such as for example to AUTOLOAD at compile
863 time rather than normal, deferred loading. See L<perlcc>. Setting
864 C<$^C = 1> is similar to calling C<B::minus_c>.
870 The current value of the debugging flags. (Mnemonic: value of B<-D>
877 The maximum system file descriptor, ordinarily 2. System file
878 descriptors are passed to exec()ed processes, while higher file
879 descriptors are not. Also, during an open(), system file descriptors are
880 preserved even if the open() fails. (Ordinary file descriptors are
881 closed before the open() is attempted.) The close-on-exec
882 status of a file descriptor will be decided according to the value of
883 C<$^F> when the corresponding file, pipe, or socket was opened, not the
888 WARNING: This variable is strictly for internal use only. Its availability,
889 behavior, and contents are subject to change without notice.
891 This variable contains compile-time hints for the Perl interpreter. At the
892 end of compilation of a BLOCK the value of this variable is restored to the
893 value when the interpreter started to compile the BLOCK.
895 When perl begins to parse any block construct that provides a lexical scope
896 (e.g., eval body, required file, subroutine body, loop body, or conditional
897 block), the existing value of $^H is saved, but its value is left unchanged.
898 When the compilation of the block is completed, it regains the saved value.
899 Between the points where its value is saved and restored, code that
900 executes within BEGIN blocks is free to change the value of $^H.
902 This behavior provides the semantic of lexical scoping, and is used in,
903 for instance, the C<use strict> pragma.
905 The contents should be an integer; different bits of it are used for
906 different pragmatic flags. Here's an example:
908 sub add_100 { $^H |= 0x100 }
915 Consider what happens during execution of the BEGIN block. At this point
916 the BEGIN block has already been compiled, but the body of foo() is still
917 being compiled. The new value of $^H will therefore be visible only while
918 the body of foo() is being compiled.
920 Substitution of the above BEGIN block with:
922 BEGIN { require strict; strict->import('vars') }
924 demonstrates how C<use strict 'vars'> is implemented. Here's a conditional
925 version of the same lexical pragma:
927 BEGIN { require strict; strict->import('vars') if $condition }
931 WARNING: This variable is strictly for internal use only. Its availability,
932 behavior, and contents are subject to change without notice.
934 The %^H hash provides the same scoping semantic as $^H. This makes it
935 useful for implementation of lexically scoped pragmas.
941 The current value of the inplace-edit extension. Use C<undef> to disable
942 inplace editing. (Mnemonic: value of B<-i> switch.)
946 By default, running out of memory is an untrappable, fatal error.
947 However, if suitably built, Perl can use the contents of C<$^M>
948 as an emergency memory pool after die()ing. Suppose that your Perl
949 were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
952 $^M = 'a' x (1 << 16);
954 would allocate a 64K buffer for use in an emergency. See the
955 F<INSTALL> file in the Perl distribution for information on how to
956 enable this option. To discourage casual use of this advanced
957 feature, there is no L<English|English> long name for this variable.
963 The name of the operating system under which this copy of Perl was
964 built, as determined during the configuration process. The value
965 is identical to C<$Config{'osname'}>. See also L<Config> and the
966 B<-V> command-line switch documented in L<perlrun>.
972 The internal variable for debugging support. The meanings of the
973 various bits are subject to change, but currently indicate:
979 Debug subroutine enter/exit.
983 Line-by-line debugging.
987 Switch off optimizations.
991 Preserve more data for future interactive inspections.
995 Keep info about source lines on which a subroutine is defined.
999 Start with single-step on.
1003 Use subroutine address instead of name when reporting.
1007 Report C<goto &subroutine> as well.
1011 Provide informative "file" names for evals based on the place they were compiled.
1015 Provide informative names to anonymous subroutines based on the place they
1020 Some bits may be relevant at compile-time only, some at
1021 run-time only. This is a new mechanism and the details may change.
1023 =item $LAST_REGEXP_CODE_RESULT
1027 The result of evaluation of the last successful C<(?{ code })>
1028 regular expression assertion (see L<perlre>). May be written to.
1030 =item $EXCEPTIONS_BEING_CAUGHT
1034 Current state of the interpreter. Undefined if parsing of the current
1035 module/eval is not finished (may happen in $SIG{__DIE__} and
1036 $SIG{__WARN__} handlers). True if inside an eval(), otherwise false.
1042 The time at which the program began running, in seconds since the
1043 epoch (beginning of 1970). The values returned by the B<-M>, B<-A>,
1044 and B<-C> filetests are based on this value.
1048 Reflects if taint mode is on or off (i.e. if the program was run with
1049 B<-T> or not). True for on, false for off.
1055 The revision, version, and subversion of the Perl interpreter, represented
1056 as a string composed of characters with those ordinals. Thus in Perl v5.6.0
1057 it equals C<chr(5) . chr(6) . chr(0)> and will return true for
1058 C<$^V eq v5.6.0>. Note that the characters in this string value can
1059 potentially be in Unicode range.
1061 This can be used to determine whether the Perl interpreter executing a
1062 script is in the right range of versions. (Mnemonic: use ^V for Version
1065 warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
1067 See the documentation of C<use VERSION> and C<require VERSION>
1068 for a convenient way to fail if the running Perl interpreter is too old.
1070 See also C<$]> for an older representation of the Perl version.
1076 The current value of the warning switch, initially true if B<-w>
1077 was used, false otherwise, but directly modifiable. (Mnemonic:
1078 related to the B<-w> switch.) See also L<warnings>.
1080 =item ${^WARNING_BITS}
1082 The current set of warning checks enabled by the C<use warnings> pragma.
1083 See the documentation of C<warnings> for more details.
1085 =item ${^WIDE_SYSTEM_CALLS}
1087 Global flag that enables system calls made by Perl to use wide character
1088 APIs native to the system, if available. This is currently only implemented
1089 on the Windows platform.
1091 This can also be enabled from the command line using the C<-C> switch.
1093 The initial value is typically C<0> for compatibility with Perl versions
1094 earlier than 5.6, but may be automatically set to C<1> by Perl if the system
1095 provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).
1097 The C<bytes> pragma always overrides the effect of this flag in the current
1098 lexical scope. See L<bytes>.
1100 =item $EXECUTABLE_NAME
1104 The name that the Perl binary itself was executed as, from C's C<argv[0]>.
1105 This may not be a full pathname, nor even necessarily in your path.
1109 The special filehandle that iterates over command-line filenames in
1110 C<@ARGV>. Usually written as the null filehandle in the angle operator
1111 C<< <> >>. Note that currently C<ARGV> only has its magical effect
1112 within the C<< <> >> operator; elsewhere it is just a plain filehandle
1113 corresponding to the last file opened by C<< <> >>. In particular,
1114 passing C<\*ARGV> as a parameter to a function that expects a filehandle
1115 may not cause your function to automatically read the contents of all the
1120 contains the name of the current file when reading from <>.
1124 The array @ARGV contains the command-line arguments intended for
1125 the script. C<$#ARGV> is generally the number of arguments minus
1126 one, because C<$ARGV[0]> is the first argument, I<not> the program's
1127 command name itself. See C<$0> for the command name.
1131 The array @F contains the fields of each line read in when autosplit
1132 mode is turned on. See L<perlrun> for the B<-a> switch. This array
1133 is package-specific, and must be declared or given a full package name
1134 if not in package main when running under C<strict 'vars'>.
1138 The array @INC contains the list of places that the C<do EXPR>,
1139 C<require>, or C<use> constructs look for their library files. It
1140 initially consists of the arguments to any B<-I> command-line
1141 switches, followed by the default Perl library, probably
1142 F</usr/local/lib/perl>, followed by ".", to represent the current
1143 directory. If you need to modify this at runtime, you should use
1144 the C<use lib> pragma to get the machine-dependent library properly
1147 use lib '/mypath/libdir/';
1150 You can also insert hooks into the file inclusion system by putting Perl
1151 code directly into @INC. Those hooks may be subroutine references, array
1152 references or blessed objects. See L<perlfunc/require> for details.
1156 Within a subroutine the array @_ contains the parameters passed to that
1157 subroutine. See L<perlsub>.
1161 The hash %INC contains entries for each filename included via the
1162 C<do>, C<require>, or C<use> operators. The key is the filename
1163 you specified (with module names converted to pathnames), and the
1164 value is the location of the file found. The C<require>
1165 operator uses this hash to determine whether a particular file has
1166 already been included.
1168 If the file was loaded via a hook (e.g. a subroutine reference, see
1169 L<perlfunc/require> for a description of these hooks), this hook is
1170 by default inserted into %INC in place of a filename. Note, however,
1171 that the hook may have set the %INC entry by itself to provide some more
1178 The hash %ENV contains your current environment. Setting a
1179 value in C<ENV> changes the environment for any child processes
1180 you subsequently fork() off.
1186 The hash %SIG contains signal handlers for signals. For example:
1188 sub handler { # 1st argument is signal name
1190 print "Caught a SIG$sig--shutting down\n";
1195 $SIG{'INT'} = \&handler;
1196 $SIG{'QUIT'} = \&handler;
1198 $SIG{'INT'} = 'DEFAULT'; # restore default action
1199 $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
1201 Using a value of C<'IGNORE'> usually has the effect of ignoring the
1202 signal, except for the C<CHLD> signal. See L<perlipc> for more about
1205 Here are some other examples:
1207 $SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
1208 $SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
1209 $SIG{"PIPE"} = *Plumber; # somewhat esoteric
1210 $SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
1212 Be sure not to use a bareword as the name of a signal handler,
1213 lest you inadvertently call it.
1215 If your system has the sigaction() function then signal handlers are
1216 installed using it. This means you get reliable signal handling. If
1217 your system has the SA_RESTART flag it is used when signals handlers are
1218 installed. This means that system calls for which restarting is supported
1219 continue rather than returning when a signal arrives. If you want your
1220 system calls to be interrupted by signal delivery then do something like
1223 use POSIX ':signal_h';
1226 sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
1227 or die "Error setting SIGALRM handler: $!\n";
1231 Certain internal hooks can be also set using the %SIG hash. The
1232 routine indicated by C<$SIG{__WARN__}> is called when a warning message is
1233 about to be printed. The warning message is passed as the first
1234 argument. The presence of a __WARN__ hook causes the ordinary printing
1235 of warnings to STDERR to be suppressed. You can use this to save warnings
1236 in a variable, or turn warnings into fatal errors, like this:
1238 local $SIG{__WARN__} = sub { die $_[0] };
1241 The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
1242 is about to be thrown. The error message is passed as the first
1243 argument. When a __DIE__ hook routine returns, the exception
1244 processing continues as it would have in the absence of the hook,
1245 unless the hook routine itself exits via a C<goto>, a loop exit, or a die().
1246 The C<__DIE__> handler is explicitly disabled during the call, so that you
1247 can die from a C<__DIE__> handler. Similarly for C<__WARN__>.
1249 Due to an implementation glitch, the C<$SIG{__DIE__}> hook is called
1250 even inside an eval(). Do not use this to rewrite a pending exception
1251 in C<$@>, or as a bizarre substitute for overriding CORE::GLOBAL::die().
1252 This strange action at a distance may be fixed in a future release
1253 so that C<$SIG{__DIE__}> is only called if your program is about
1254 to exit, as was the original intent. Any other use is deprecated.
1256 C<__DIE__>/C<__WARN__> handlers are very special in one respect:
1257 they may be called to report (probable) errors found by the parser.
1258 In such a case the parser may be in inconsistent state, so any
1259 attempt to evaluate Perl code from such a handler will probably
1260 result in a segfault. This means that warnings or errors that
1261 result from parsing Perl should be used with extreme caution, like
1264 require Carp if defined $^S;
1265 Carp::confess("Something wrong") if defined &Carp::confess;
1266 die "Something wrong, but could not load Carp to give backtrace...
1267 To see backtrace try starting Perl with -MCarp switch";
1269 Here the first line will load Carp I<unless> it is the parser who
1270 called the handler. The second line will print backtrace and die if
1271 Carp was available. The third line will be executed only if Carp was
1274 See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
1275 L<warnings> for additional information.
1279 =head2 Error Indicators
1281 The variables C<$@>, C<$!>, C<$^E>, and C<$?> contain information
1282 about different types of error conditions that may appear during
1283 execution of a Perl program. The variables are shown ordered by
1284 the "distance" between the subsystem which reported the error and
1285 the Perl process. They correspond to errors detected by the Perl
1286 interpreter, C library, operating system, or an external program,
1289 To illustrate the differences between these variables, consider the
1290 following Perl expression, which uses a single-quoted string:
1293 open my $pipe, "/cdrom/install |" or die $!;
1295 close $pipe or die "bad pipe: $?, $!";
1298 After execution of this statement all 4 variables may have been set.
1300 C<$@> is set if the string to be C<eval>-ed did not compile (this
1301 may happen if C<open> or C<close> were imported with bad prototypes),
1302 or if Perl code executed during evaluation die()d . In these cases
1303 the value of $@ is the compile error, or the argument to C<die>
1304 (which will interpolate C<$!> and C<$?>!). (See also L<Fatal>,
1307 When the eval() expression above is executed, open(), C<< <PIPE> >>,
1308 and C<close> are translated to calls in the C run-time library and
1309 thence to the operating system kernel. C<$!> is set to the C library's
1310 C<errno> if one of these calls fails.
1312 Under a few operating systems, C<$^E> may contain a more verbose
1313 error indicator, such as in this case, "CDROM tray not closed."
1314 Systems that do not support extended error messages leave C<$^E>
1317 Finally, C<$?> may be set to non-0 value if the external program
1318 F</cdrom/install> fails. The upper eight bits reflect specific
1319 error conditions encountered by the program (the program's exit()
1320 value). The lower eight bits reflect mode of failure, like signal
1321 death and core dump information See wait(2) for details. In
1322 contrast to C<$!> and C<$^E>, which are set only if error condition
1323 is detected, the variable C<$?> is set on each C<wait> or pipe
1324 C<close>, overwriting the old value. This is more like C<$@>, which
1325 on every eval() is always set on failure and cleared on success.
1327 For more details, see the individual descriptions at C<$@>, C<$!>, C<$^E>,
1330 =head2 Technical Note on the Syntax of Variable Names
1332 Variable names in Perl can have several formats. Usually, they
1333 must begin with a letter or underscore, in which case they can be
1334 arbitrarily long (up to an internal limit of 251 characters) and
1335 may contain letters, digits, underscores, or the special sequence
1336 C<::> or C<'>. In this case, the part before the last C<::> or
1337 C<'> is taken to be a I<package qualifier>; see L<perlmod>.
1339 Perl variable names may also be a sequence of digits or a single
1340 punctuation or control character. These names are all reserved for
1341 special uses by Perl; for example, the all-digits names are used
1342 to hold data captured by backreferences after a regular expression
1343 match. Perl has a special syntax for the single-control-character
1344 names: It understands C<^X> (caret C<X>) to mean the control-C<X>
1345 character. For example, the notation C<$^W> (dollar-sign caret
1346 C<W>) is the scalar variable whose name is the single character
1347 control-C<W>. This is better than typing a literal control-C<W>
1350 Finally, new in Perl 5.6, Perl variable names may be alphanumeric
1351 strings that begin with control characters (or better yet, a caret).
1352 These variables must be written in the form C<${^Foo}>; the braces
1353 are not optional. C<${^Foo}> denotes the scalar variable whose
1354 name is a control-C<F> followed by two C<o>'s. These variables are
1355 reserved for future special uses by Perl, except for the ones that
1356 begin with C<^_> (control-underscore or caret-underscore). No
1357 control-character name that begins with C<^_> will acquire a special
1358 meaning in any future version of Perl; such names may therefore be
1359 used safely in programs. C<$^_> itself, however, I<is> reserved.
1361 Perl identifiers that begin with digits, control characters, or
1362 punctuation characters are exempt from the effects of the C<package>
1363 declaration and are always forced to be in package C<main>. A few
1364 other names are also exempt:
1372 In particular, the new special C<${^_XYZ}> variables are always taken
1373 to be in package C<main>, regardless of any C<package> declarations
1378 Due to an unfortunate accident of Perl's implementation, C<use
1379 English> imposes a considerable performance penalty on all regular
1380 expression matches in a program, regardless of whether they occur
1381 in the scope of C<use English>. For that reason, saying C<use
1382 English> in libraries is strongly discouraged. See the
1383 Devel::SawAmpersand module documentation from CPAN
1384 (http://www.perl.com/CPAN/modules/by-module/Devel/)
1385 for more information.
1387 Having to even think about the C<$^S> variable in your exception
1388 handlers is simply wrong. C<$SIG{__DIE__}> as currently implemented
1389 invites grievous and difficult to track down errors. Avoid it
1390 and use an C<END{}> or CORE::GLOBAL::die override instead.