PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)
perlvar - Perl predefined variables
Predefined Names
The following names have special meaning to Perl. Most
punctuation names have reasonable mnemonics, or analogs in
the shells. Nevertheless, if you wish to use long variable
names, you need only say
use English;
at the top of your program. This aliases all the short names
to the long names in the current package. Some even have
medium names, generally borrowed from awk. In general, it's
best to use the
use English '-no_match_vars';
invocation if you don't need $PREMATCH, $MATCH, or $POST-
MATCH, as it avoids a certain performance hit with the use
of regular expressions. See English.
Variables that depend on the currently selected filehandle
may be set by calling an appropriate object method on the
IO::Handle object, although this is less efficient than
using the regular built-in variables. (Summary lines below
for this contain the word HANDLE.) First you must say
use IO::Handle;
after which you may use either
method HANDLE EXPR
or more safely,
HANDLE->method(EXPR)
Each method returns the old value of the IO::Handle attri-
bute. The methods each take an optional EXPR, which, if sup-
plied, specifies the new value for the IO::Handle attribute
in question. If not supplied, most methods do nothing to
the current value--except for autoflush(), which will assume
a 1 for you, just to be different.
Because loading in the IO::Handle class is an expensive
operation, you should learn how to use the regular built-in
variables.
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A few of these variables are considered "read-only". This
means that if you try to assign to this variable, either
directly or indirectly through a reference, you'll raise a
run-time exception.
You should be very careful when modifying the default values
of most special variables described in this document. In
most cases you want to localize these variables before
changing them, since if you don't, the change may affect
other modules which rely on the default values of the spe-
cial variables that you have changed. This is one of the
correct ways to read the whole file at once:
open my $fh, "foo" or die $!;
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;
But the following code is quite bad:
open my $fh, "foo" or die $!;
undef $/; # enable slurp mode
my $content = <$fh>;
close $fh;
since some other module, may want to read data from some
file in the default "line mode", so if the code we have just
presented has been executed, the global value of $/ is now
changed for any other code running inside the same Perl
interpreter.
Usually when a variable is localized you want to make sure
that this change affects the shortest scope possible. So
unless you are already inside some short "{}" block, you
should create one yourself. For example:
my $content = '';
open my $fh, "foo" or die $!;
{
local $/;
$content = <$fh>;
}
close $fh;
Here is an example of how your own code can go broken:
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for (1..5){
nasty_break();
print "$_ ";
}
sub nasty_break {
$_ = 5;
# do something with $_
}
You probably expect this code to print:
1 2 3 4 5
but instead you get:
5 5 5 5 5
Why? Because nasty_break() modifies $_ without localizing it
first. The fix is to add local():
local $_ = 5;
It's easy to notice the problem in such a short example, but
in more complicated code you are looking for trouble if you
don't localize changes to the special variables.
The following list is ordered by scalar variables first,
then the arrays, then the hashes.
$ARG
$_ The default input and pattern-searching space. The
following pairs are equivalent:
while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chomp
chomp($_)
Here are the places where Perl will assume $_ even
if you don't use it:
* Various unary functions, including functions like
ord() and int(), as well as the all file tests
("-f", "-d") except for "-t", which defaults to
STDIN.
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* Various list functions like print() and unlink().
* The pattern matching operations "m//", "s///",
and "tr///" when used without an "=~" operator.
* The default iterator variable in a "foreach" loop
if no other variable is supplied.
* The implicit iterator variable in the grep() and
map() functions.
* The default place to put an input record when a
"<FH>" operation's result is tested by itself as
the sole criterion of a "while" test. Outside a
"while" test, this will not happen.
(Mnemonic: underline is understood in certain opera-
tions.)
$a
$b Special package variables when using sort(), see
"sort" in perlfunc. Because of this specialness $a
and $b don't need to be declared (using use vars, or
our()) even when using the "strict 'vars'" pragma.
Don't lexicalize them with "my $a" or "my $b" if you
want to be able to use them in the sort() comparison
block or function.
$<digits>
Contains the subpattern from the corresponding set
of capturing parentheses from the last pattern
match, not counting patterns matched in nested
blocks that have been exited already. (Mnemonic:
like \digits.) These variables are all read-only
and dynamically scoped to the current BLOCK.
$MATCH
$& The string matched by the last successful pattern
match (not counting any matches hidden within a
BLOCK or eval() enclosed by the current BLOCK).
(Mnemonic: like & in some editors.) This variable
is read-only and dynamically scoped to the current
BLOCK.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See "BUGS".
$PREMATCH
$` The string preceding whatever was matched by the
last successful pattern match (not counting any
matches hidden within a BLOCK or eval enclosed by
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the current BLOCK). (Mnemonic: "`" often precedes a
quoted string.) This variable is read-only.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See "BUGS".
$POSTMATCH
$' The string following whatever was matched by the
last successful pattern match (not counting any
matches hidden within a BLOCK or eval() enclosed by
the current BLOCK). (Mnemonic: "'" often follows a
quoted string.) Example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
This variable is read-only and dynamically scoped to
the current BLOCK.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See "BUGS".
$LAST_PAREN_MATCH
$+ The text matched by the last bracket of the last
successful search pattern. This is useful if you
don't know which one of a set of alternative pat-
terns matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This
variable is read-only and dynamically scoped to the
current BLOCK.
$^N The text matched by the used group most-recently
closed (i.e. the group with the rightmost closing
parenthesis) of the last successful search pattern.
(Mnemonic: the (possibly) Nested parenthesis that
most recently closed.)
This is primarily used inside "(?{...})" blocks for
examining text recently matched. For example, to
effectively capture text to a variable (in addition
to $1, $2, etc.), replace "(...)" with
(?:(...)(?{ $var = $^N }))
By setting and then using $var in this way relieves
you from having to worry about exactly which
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numbered set of parentheses they are.
This variable is dynamically scoped to the current
BLOCK.
@LAST_MATCH_END
@+ This array holds the offsets of the ends of the last
successful submatches in the currently active
dynamic scope. $+[0] is the offset into the string
of the end of the entire match. This is the same
value as what the "pos" function returns when called
on the variable that was matched against. The nth
element of this array holds the offset of the nth
submatch, so $+[1] is the offset past where $1 ends,
$+[2] the offset past where $2 ends, and so on. You
can use $#+ to determine how many subgroups were in
the last successful match. See the examples given
for the "@-" variable.
$* Set to a non-zero integer value to do multi-line
matching within a string, 0 (or undefined) to tell
Perl that it can assume that strings contain a sin-
gle line, for the purpose of optimizing pattern
matches. Pattern matches on strings containing mul-
tiple newlines can produce confusing results when $*
is 0 or undefined. Default is undefined. (Mnemonic:
* matches multiple things.) This variable influences
the interpretation of only "^" and "$". A literal
newline can be searched for even when "$* == 0".
Use of $* is deprecated in modern Perl, supplanted
by the "/s" and "/m" modifiers on pattern matching.
Assigning a non-numerical value to $* triggers a
warning (and makes $* act if "$* == 0"), while
assigning a numerical value to $* makes that an
implicit "int" is applied on the value.
HANDLE->input_line_number(EXPR)
$INPUT_LINE_NUMBER
$NR
$. Current line number for the last filehandle
accessed.
Each filehandle in Perl counts the number of lines
that have been read from it. (Depending on the
value of $/, Perl's idea of what constitutes a line
may not match yours.) When a line is read from a
filehandle (via readline() or "<>"), or when tell()
or seek() is called on it, $. becomes an alias to
the line counter for that filehandle.
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You can adjust the counter by assigning to $., but
this will not actually move the seek pointer.
Localizing $. will not localize the filehandle's
line count. Instead, it will localize perl's notion
of which filehandle $. is currently aliased to.
$. is reset when the filehandle is closed, but not
when an open filehandle is reopened without an
intervening close(). For more details, see "I/O
Operators" in perlop. Because "<>" never does an
explicit close, line numbers increase across ARGV
files (but see examples in "eof" in perlfunc).
You can also use "HANDLE->input_line_number(EXPR)"
to access the line counter for a given filehandle
without having to worry about which handle you last
accessed.
(Mnemonic: many programs use "." to mean the current
line number.)
IO::Handle->input_record_separator(EXPR)
$INPUT_RECORD_SEPARATOR
$RS
$/ The input record separator, newline by default.
This influences Perl's idea of what a "line" is.
Works like awk's RS variable, including treating
empty lines as a terminator if set to the null
string. (An empty line cannot contain any spaces or
tabs.) You may set it to a multi-character string
to match a multi-character terminator, or to "undef"
to read through the end of file. Setting it to
"\n\n" means something slightly different than set-
ting to "", if the file contains consecutive empty
lines. Setting to "" will treat two or more con-
secutive empty lines as a single empty line. Set-
ting to "\n\n" will blindly assume that the next
input character belongs to the next paragraph, even
if it's a newline. (Mnemonic: / delimits line boun-
daries when quoting poetry.)
local $/; # enable "slurp" mode
local $_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
Remember: the value of $/ is a string, not a regex.
awk has to be better for something. :-)
Setting $/ to a reference to an integer, scalar con-
taining an integer, or scalar that's convertible to
an integer will attempt to read records instead of
lines, with the maximum record size being the
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referenced integer. So this:
local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, $myfile or die $!;
local $_ = <$fh>;
will read a record of no more than 32768 bytes from
FILE. If you're not reading from a record-oriented
file (or your OS doesn't have record-oriented
files), then you'll likely get a full chunk of data
with every read. If a record is larger than the
record size you've set, you'll get the record back
in pieces.
On VMS, record reads are done with the equivalent of
"sysread", so it's best not to mix record and non-
record reads on the same file. (This is unlikely to
be a problem, because any file you'd want to read in
record mode is probably unusable in line mode.) Non-
VMS systems do normal I/O, so it's safe to mix
record and non-record reads of a file.
See also "Newlines" in perlport. Also see $..
HANDLE->autoflush(EXPR)
$OUTPUT_AUTOFLUSH
$| If set to nonzero, forces a flush right away and
after every write or print on the currently selected
output channel. Default is 0 (regardless of whether
the channel is really buffered by the system or not;
$| tells you only whether you've asked Perl expli-
citly to flush after each write). STDOUT will typi-
cally be line buffered if output is to the terminal
and block buffered otherwise. Setting this variable
is useful primarily when you are outputting to a
pipe or socket, such as when you are running a Perl
program under rsh and want to see the output as it's
happening. This has no effect on input buffering.
See "getc" in perlfunc for that. (Mnemonic: when
you want your pipes to be piping hot.)
IO::Handle->output_field_separator EXPR
$OUTPUT_FIELD_SEPARATOR
$OFS
$, The output field separator for the print operator.
If defined, this value is printed between each of
print's arguments. Default is "undef". (Mnemonic:
what is printed when there is a "," in your print
statement.)
IO::Handle->output_record_separator EXPR
$OUTPUT_RECORD_SEPARATOR
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$ORS
$\ The output record separator for the print operator.
If defined, this value is printed after the last of
print's arguments. Default is "undef". (Mnemonic:
you set "$\" instead of adding "\n" at the end of
the print. Also, it's just like $/, but it's what
you get "back" from Perl.)
$LIST_SEPARATOR
$" This is like $, except that it applies to array and
slice values interpolated into a double-quoted
string (or similar interpreted string). Default is
a space. (Mnemonic: obvious, I think.)
$SUBSCRIPT_SEPARATOR
$SUBSEP
$; The subscript separator for multidimensional array
emulation. If you refer to a hash element as
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
But don't put
@foo{$a,$b,$c} # a slice--note the @
which means
($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk. If
your keys contain binary data there might not be any
safe value for $;. (Mnemonic: comma (the syntactic
subscript separator) is a semi-semicolon. Yeah, I
know, it's pretty lame, but $, is already taken for
something more important.)
Consider using "real" multidimensional arrays as
described in perllol.
$# The output format for printed numbers. This vari-
able is a half-hearted attempt to emulate awk's OFMT
variable. There are times, however, when awk and
Perl have differing notions of what counts as
numeric. The initial value is "%.ng", where n is
the value of the macro DBL_DIG from your system's
float.h. This is different from awk's default OFMT
setting of "%.6g", so you need to set $# explicitly
to get awk's value. (Mnemonic: # is the number
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sign.)
Use of $# is deprecated.
HANDLE->format_page_number(EXPR)
$FORMAT_PAGE_NUMBER
$% The current page number of the currently selected
output channel. Used with formats. (Mnemonic: % is
page number in nroff.)
HANDLE->format_lines_per_page(EXPR)
$FORMAT_LINES_PER_PAGE
$= The current page length (printable lines) of the
currently selected output channel. Default is 60.
Used with formats. (Mnemonic: = has horizontal
lines.)
HANDLE->format_lines_left(EXPR)
$FORMAT_LINES_LEFT
$- The number of lines left on the page of the
currently selected output channel. Used with for-
mats. (Mnemonic: lines_on_page - lines_printed.)
@LAST_MATCH_START
@- $-[0] is the offset of the start of the last suc-
cessful match. "$-["n"]" is the offset of the start
of the substring matched by n-th subpattern, or
undef if the subpattern did not match.
Thus after a match against $_, $& coincides with
"substr $_, $-[0], $+[0] - $-[0]". Similarly, $n
coincides with "substr $_, $-[n], $+[n] - $-[n]" if
"$-[n]" is defined, and $+ coincides with "substr
$_, $-[$#-], $+[$#-] - $-[$#-]". One can use "$#-"
to find the last matched subgroup in the last suc-
cessful match. Contrast with $#+, the number of
subgroups in the regular expression. Compare with
"@+".
This array holds the offsets of the beginnings of
the last successful submatches in the currently
active dynamic scope. "$-[0]" is the offset into the
string of the beginning of the entire match. The
nth element of this array holds the offset of the
nth submatch, so "$-[1]" is the offset where $1
begins, "$-[2]" the offset where $2 begins, and so
on.
After a match against some variable $var:
$` is the same as "substr($var, 0, $-[0])"
$& is the same as "substr($var, $-[0], $+[0] - $-[0])"
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$' is the same as "substr($var, $+[0])"
$1 is the same as "substr($var, $-[1], $+[1] - $-[1])"
$2 is the same as "substr($var, $-[2], $+[2] - $-[2])"
$3 is the same as "substr($var, $-[3], $+[3] - $-[3])"
HANDLE->format_name(EXPR)
$FORMAT_NAME
$~ The name of the current report format for the
currently selected output channel. Default is the
name of the filehandle. (Mnemonic: brother to $^.)
HANDLE->format_top_name(EXPR)
$FORMAT_TOP_NAME
$^ The name of the current top-of-page format for the
currently selected output channel. Default is the
name of the filehandle with _TOP appended.
(Mnemonic: points to top of page.)
IO::Handle->format_line_break_characters EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$: The current set of characters after which a string
may be broken to fill continuation fields (starting
with ^) in a format. Default is " \n-", to break on
whitespace or hyphens. (Mnemonic: a "colon" in poe-
try is a part of a line.)
IO::Handle->format_formfeed EXPR
$FORMAT_FORMFEED
$^L What formats output as a form feed. Default is \f.
$ACCUMULATOR
$^A The current value of the write() accumulator for
format() lines. A format contains formline() calls
that put their result into $^A. After calling its
format, write() prints out the contents of $^A and
empties. So you never really see the contents of $^A
unless you call formline() yourself and then look at
it. See perlform and "formline()" in perlfunc.
$CHILD_ERROR
$? The status returned by the last pipe close, backtick
(``) command, successful call to wait() or wait-
pid(), or from the system() operator. This is just
the 16-bit status word returned by the wait() system
call (or else is made up to look like it). Thus,
the exit value of the subprocess is really ("$? >>
8"), and "$? & 127" gives which signal, if any, the
process died from, and "$? & 128" reports whether
there was a core dump. (Mnemonic: similar to sh and
ksh.)
Additionally, if the "h_errno" variable is supported
in C, its value is returned via $? if any
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"gethost*()" function fails.
If you have installed a signal handler for
"SIGCHLD", the value of $? will usually be wrong
outside that handler.
Inside an "END" subroutine $? contains the value
that is going to be given to "exit()". You can
modify $? in an "END" subroutine to change the exit
status of your program. For example:
END {
$? = 1 if $? == 255; # die would make it 255
}
Under VMS, the pragma "use vmsish 'status'" makes $?
reflect the actual VMS exit status, instead of the
default emulation of POSIX status; see "$?" in
perlvms for details.
Also see "Error Indicators".
${^ENCODING}
The object reference to the Encode object that is
used to convert the source code to Unicode. Thanks
to this variable your perl script does not have to
be written in UTF-8. Default is undef. The direct
manipulation of this variable is highly discouraged.
See encoding for more details.
$OS_ERROR
$ERRNO
$! If used numerically, yields the current value of the
C "errno" variable, or in other words, if a system
or library call fails, it sets this variable. This
means that the value of $! is meaningful only
immediately after a failure:
if (open(FH, $filename)) {
# Here $! is meaningless.
...
} else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# here $! is meaningless.
In the above meaningless stands for anything: zero,
non-zero, "undef". A successful system or library
call does not set the variable to zero.
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If used as a string, yields the corresponding system
error string. You can assign a number to $! to set
errno if, for instance, you want "$!" to return the
string for error n, or you want to set the exit
value for the die() operator. (Mnemonic: What just
went bang?)
Also see "Error Indicators".
%! Each element of "%!" has a true value only if $! is
set to that value. For example, $!{ENOENT} is true
if and only if the current value of $! is "ENOENT";
that is, if the most recent error was "No such file
or directory" (or its moral equivalent: not all
operating systems give that exact error, and cer-
tainly not all languages). To check if a particular
key is meaningful on your system, use "exists
$!{the_key}"; for a list of legal keys, use "keys
%!". See Errno for more information, and also see
above for the validity of $!.
$EXTENDED_OS_ERROR
$^E Error information specific to the current operating
system. At the moment, this differs from $! under
only VMS, OS/2, and Win32 (and for MacPerl). On all
other platforms, $^E is always just the same as $!.
Under VMS, $^E provides the VMS status value from
the last system error. This is more specific infor-
mation about the last system error than that pro-
vided by $!. This is particularly important when $!
is set to EVMSERR.
Under OS/2, $^E is set to the error code of the last
call to OS/2 API either via CRT, or directly from
perl.
Under Win32, $^E always returns the last error
information reported by the Win32 call "GetLastEr-
ror()" which describes the last error from within
the Win32 API. Most Win32-specific code will report
errors via $^E. ANSI C and Unix-like calls set
"errno" and so most portable Perl code will report
errors via $!.
Caveats mentioned in the description of $! generally
apply to $^E, also. (Mnemonic: Extra error explana-
tion.)
Also see "Error Indicators".
$EVAL_ERROR
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$@ The Perl syntax error message from the last eval()
operator. If $@ is the null string, the last eval()
parsed and executed correctly (although the opera-
tions you invoked may have failed in the normal
fashion). (Mnemonic: Where was the syntax error
"at"?)
Warning messages are not collected in this variable.
You can, however, set up a routine to process warn-
ings by setting $SIG{__WARN__} as described below.
Also see "Error Indicators".
$PROCESS_ID
$PID
$$ The process number of the Perl running this script.
You should consider this variable read-only,
although it will be altered across fork() calls.
(Mnemonic: same as shells.)
Note for Linux users: on Linux, the C functions
"getpid()" and "getppid()" return different values
from different threads. In order to be portable,
this behavior is not reflected by $$, whose value
remains consistent across threads. If you want to
call the underlying "getpid()", you may use the CPAN
module "Linux::Pid".
$REAL_USER_ID
$UID
$< The real uid of this process. (Mnemonic: it's the
uid you came from, if you're running setuid.) You
can change both the real uid and the effective uid
at the same time by using POSIX::setuid(). Since
changes to $< require a system call, check $! after
a change attempt to detect any possible errors.
$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. Example:
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
You can change both the effective uid and the real
uid at the same time by using POSIX::setuid().
Changes to $> require a check to $! to detect any
possible errors after an attempted change.
(Mnemonic: it's the uid you went to, if you're run-
ning setuid.) $< and $> can be swapped only on
machines supporting setreuid().
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$REAL_GROUP_ID
$GID
$( The real gid of this process. If you are on a
machine that supports membership in multiple groups
simultaneously, gives a space separated list of
groups you are in. The first number is the one
returned by getgid(), and the subsequent ones by
getgroups(), one of which may be the same as the
first number.
However, a value assigned to $( must be a single
number used to set the real gid. So the value given
by $( should not be assigned back to $( without
being forced numeric, such as by adding zero.
You can change both the real gid and the effective
gid at the same time by using POSIX::setgid().
Changes to $( require a check to $! to detect any
possible errors after an attempted change.
(Mnemonic: parentheses are used to group things.
The real gid is the group you left, if you're run-
ning setgid.)
$EFFECTIVE_GROUP_ID
$EGID
$) The effective gid of this process. If you are on a
machine that supports membership in multiple groups
simultaneously, gives a space separated list of
groups you are in. The first number is the one
returned by getegid(), and the subsequent ones by
getgroups(), one of which may be the same as the
first number.
Similarly, a value assigned to $) must also be a
space-separated list of numbers. The first number
sets the effective gid, and the rest (if any) are
passed to setgroups(). To get the effect of an
empty list for setgroups(), just repeat the new
effective gid; that is, to force an effective gid of
5 and an effectively empty setgroups() list, say "
$) = "5 5" ".
You can change both the effective gid and the real
gid at the same time by using POSIX::setgid() (use
only a single numeric argument). Changes to $)
require a check to $! to detect any possible errors
after an attempted change.
(Mnemonic: parentheses are used to group things.
The effective gid is the group that's right for you,
if you're running setgid.)
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$<, $>, $( and $) can be set only on machines that
support the corresponding set[re][ug]id() routine.
$( and $) can be swapped only on machines supporting
setregid().
$PROGRAM_NAME
$0 Contains the name of the program being executed.
On some (read: not all) operating systems assigning
to $0 modifies the argument area that the "ps" pro-
gram sees. On some platforms you may have to use
special "ps" options or a different "ps" to see the
changes. Modifying the $0 is more useful as a way
of indicating the current program state than it is
for hiding the program you're running. (Mnemonic:
same as sh and ksh.)
Note that there are platform specific limitations on
the maximum length of $0. In the most extreme case
it may be limited to the space occupied by the ori-
ginal $0.
In some platforms there may be arbitrary amount of
padding, for example space characters, after the
modified name as shown by "ps". In some platforms
this padding may extend all the way to the original
length of the argument area, no matter what you do
(this is the case for example with Linux 2.2).
Note for BSD users: setting $0 does not completely
remove "perl" from the ps(1) output. For example,
setting $0 to "foobar" may result in "perl: foobar
(perl)" (whether both the "perl: " prefix and the "
(perl)" suffix are shown depends on your exact BSD
variant and version). This is an operating system
feature, Perl cannot help it.
In multithreaded scripts Perl coordinates the
threads so that any thread may modify its copy of
the $0 and the change becomes visible to ps(1)
(assuming the operating system plays along). Note
that the view of $0 the other threads have will not
change since they have their own copies of it.
$[ The index of the first element in an array, and of
the first character in a substring. Default is 0,
but you could theoretically set it to 1 to make Perl
behave more like awk (or Fortran) when subscripting
and when evaluating the index() and substr() func-
tions. (Mnemonic: [ begins subscripts.)
As of release 5 of Perl, assignment to $[ is treated
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as a compiler directive, and cannot influence the
behavior of any other file. (That's why you can only
assign compile-time constants to it.) Its use is
highly discouraged.
Note that, unlike other compile-time directives
(such as strict), assignment to $[ can be seen from
outer lexical scopes in the same file. However, you
can use local() on it to strictly bind its value to
a lexical block.
$] The version + patchlevel / 1000 of the Perl inter-
preter. This variable can be used to determine
whether the Perl interpreter executing a script is
in the right range of versions. (Mnemonic: Is this
version of perl in the right bracket?) Example:
warn "No checksumming!\n" if $] < 3.019;
See also the documentation of "use VERSION" and
"require VERSION" for a convenient way to fail if
the running Perl interpreter is too old.
When testing the variable, to steer clear of float-
ing point inaccuracies you might want to prefer the
inequality tests "<" and ">" to the tests containing
equivalence: "<=", "==", and ">=".
The floating point representation can sometimes lead
to inaccurate numeric comparisons. See $^V for a
more modern representation of the Perl version that
allows accurate string comparisons.
$COMPILING
$^C The current value of the flag associated with the -c
switch. Mainly of use with -MO=... to allow code to
alter its behavior when being compiled, such as for
example to AUTOLOAD at compile time rather than nor-
mal, deferred loading. See perlcc. Setting "$^C =
1" is similar to calling "B::minus_c".
$DEBUGGING
$^D The current value of the debugging flags.
(Mnemonic: value of -D switch.) May be read or set.
Like its command-line equivalent, you can use
numeric or symbolic values, eg "$^D = 10" or "$^D =
"st"".
$SYSTEM_FD_MAX
$^F The maximum system file descriptor, ordinarily 2.
System file descriptors are passed to exec()ed
processes, while higher file descriptors are not.
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Also, during an open(), system file descriptors are
preserved even if the open() fails. (Ordinary file
descriptors are closed before the open() is
attempted.) The close-on-exec status of a file
descriptor will be decided according to the value of
$^F when the corresponding file, pipe, or socket was
opened, not the time of the exec().
$^H WARNING: This variable is strictly for internal use
only. Its availability, behavior, and contents are
subject to change without notice.
This variable contains compile-time hints for the
Perl interpreter. At the end of compilation of a
BLOCK the value of this variable is restored to the
value when the interpreter started to compile the
BLOCK.
When perl begins to parse any block construct that
provides a lexical scope (e.g., eval body, required
file, subroutine body, loop body, or conditional
block), the existing value of $^H is saved, but its
value is left unchanged. When the compilation of the
block is completed, it regains the saved value.
Between the points where its value is saved and
restored, code that executes within BEGIN blocks is
free to change the value of $^H.
This behavior provides the semantic of lexical scop-
ing, and is used in, for instance, the "use strict"
pragma.
The contents should be an integer; different bits of
it are used for different pragmatic flags. Here's
an example:
sub add_100 { $^H |= 0x100 }
sub foo {
BEGIN { add_100() }
bar->baz($boon);
}
Consider what happens during execution of the BEGIN
block. At this point the BEGIN block has already
been compiled, but the body of foo() is still being
compiled. The new value of $^H will therefore be
visible only while the body of foo() is being com-
piled.
Substitution of the above BEGIN block with:
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BEGIN { require strict; strict->import('vars') }
demonstrates how "use strict 'vars'" is implemented.
Here's a conditional version of the same lexical
pragma:
BEGIN { require strict; strict->import('vars') if $condition }
%^H WARNING: This variable is strictly for internal use
only. Its availability, behavior, and contents are
subject to change without notice.
The %^H hash provides the same scoping semantic as
$^H. This makes it useful for implementation of
lexically scoped pragmas.
$INPLACE_EDIT
$^I The current value of the inplace-edit extension.
Use "undef" to disable inplace editing. (Mnemonic:
value of -i switch.)
$^M By default, running out of memory is an untrappable,
fatal error. However, if suitably built, Perl can
use the contents of $^M as an emergency memory pool
after die()ing. Suppose that your Perl were com-
piled with "-DPERL_EMERGENCY_SBRK" and used Perl's
malloc. Then
$^M = 'a' x (1 << 16);
would allocate a 64K buffer for use in an emergency.
See the INSTALL file in the Perl distribution for
information on how to add custom C compilation flags
when compiling perl. To discourage casual use of
this advanced feature, there is no English long name
for this variable.
$OSNAME
$^O The name of the operating system under which this
copy of Perl was built, as determined during the
configuration process. The value is identical to
$Config{'osname'}. See also Config and the -V
command-line switch documented in perlrun.
In Windows platforms, $^O is not very helpful: since
it is always "MSWin32", it doesn't tell the differ-
ence between 95/98/ME/NT/2000/XP/CE/.NET. Use
Win32::GetOSName() or Win32::GetOSVersion() (see
Win32 and perlport) to distinguish between the vari-
ants.
${^OPEN}
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An internal variable used by PerlIO. A string in
two parts, separated by a "\0" byte, the first part
describes the input layers, the second part
describes the output layers.
$PERLDB
$^P The internal variable for debugging support. The
meanings of the various bits are subject to change,
but currently indicate:
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging.
0x04 Switch off optimizations.
0x08 Preserve more data for future interactive
inspections.
0x10 Keep info about source lines on which a sub-
routine is defined.
0x20 Start with single-step on.
0x40 Use subroutine address instead of name when
reporting.
0x80 Report "goto &subroutine" as well.
0x100 Provide informative "file" names for evals
based on the place they were compiled.
0x200 Provide informative names to anonymous subrou-
tines based on the place they were compiled.
0x400 Debug assertion subroutines enter/exit.
Some bits may be relevant at compile-time only, some
at run-time only. This is a new mechanism and the
details may change.
$LAST_REGEXP_CODE_RESULT
$^R The result of evaluation of the last successful "(?{
code })" regular expression assertion (see perlre).
May be written to.
$EXCEPTIONS_BEING_CAUGHT
$^S Current state of the interpreter.
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$^S State
--------- -------------------
undef Parsing module/eval
true (1) Executing an eval
false (0) Otherwise
The first state may happen in $SIG{__DIE__} and
$SIG{__WARN__} handlers.
$BASETIME
$^T The time at which the program began running, in
seconds since the epoch (beginning of 1970). The
values returned by the -M, -A, and -C filetests are
based on this value.
${^TAINT}
Reflects if taint mode is on or off. 1 for on (the
program was run with -T), 0 for off, -1 when only
taint warnings are enabled (i.e. with -t or -TU).
${^UNICODE}
Reflects certain Unicode settings of Perl. See
perlrun documentation for the "-C" switch for more
information about the possible values. This variable
is set during Perl startup and is thereafter
read-only.
${^UTF8LOCALE}
This variable indicates whether an UTF-8 locale was
detected by perl at startup. This information is
used by perl when it's in adjust-utf8ness-to-locale
mode (as when run with the "-CL" command-line
switch); see perlrun for more info on this.
$PERL_VERSION
$^V The revision, version, and subversion of the Perl
interpreter, represented as a string composed of
characters with those ordinals. Thus in Perl v5.6.0
it equals "chr(5) . chr(6) . chr(0)" and will return
true for "$^V eq v5.6.0". Note that the characters
in this string value can potentially be in Unicode
range.
This can be used to determine whether the Perl
interpreter executing a script is in the right range
of versions. (Mnemonic: use ^V for Version Con-
trol.) Example:
warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
To convert $^V into its string representation use
sprintf()'s "%vd" conversion:
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printf "version is v%vd\n", $^V; # Perl's version
See the documentation of "use VERSION" and "require
VERSION" for a convenient way to fail if the running
Perl interpreter is too old.
See also $] for an older representation of the Perl
version.
$WARNING
$^W The current value of the warning switch, initially
true if -w was used, false otherwise, but directly
modifiable. (Mnemonic: related to the -w switch.)
See also warnings.
${^WARNING_BITS}
The current set of warning checks enabled by the
"use warnings" pragma. See the documentation of
"warnings" for more details.
$EXECUTABLE_NAME
$^X The name used to execute the current copy of Perl,
from C's "argv[0]" or (where supported)
/proc/self/exe.
Depending on the host operating system, the value of
$^X may be a relative or absolute pathname of the
perl program file, or may be the string used to
invoke perl but not the pathname of the perl program
file. Also, most operating systems permit invoking
programs that are not in the PATH environment vari-
able, so there is no guarantee that the value of $^X
is in PATH. For VMS, the value may or may not
include a version number.
You usually can use the value of $^X to re-invoke an
independent copy of the same perl that is currently
running, e.g.,
@first_run = `$^X -le "print int rand 100 for 1..100"`;
But recall that not all operating systems support
forking or capturing of the output of commands, so
this complex statement may not be portable.
It is not safe to use the value of $^X as a path
name of a file, as some operating systems that have
a mandatory suffix on executable files do not
require use of the suffix when invoking a command.
To convert the value of $^X to a path name, use the
following statements:
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# Build up a set of file names (not command names).
use Config;
$this_perl = $^X;
if ($^O ne 'VMS')
{$this_perl .= $Config{_exe}
unless $this_perl =~ m/$Config{_exe}$/i;}
Because many operating systems permit anyone with
read access to the Perl program file to make a copy
of it, patch the copy, and then execute the copy,
the security-conscious Perl programmer should take
care to invoke the installed copy of perl, not the
copy referenced by $^X. The following statements
accomplish this goal, and produce a pathname that
can be invoked as a command or referenced as a file.
use Config;
$secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS')
{$secure_perl_path .= $Config{_exe}
unless $secure_perl_path =~ m/$Config{_exe}$/i;}
ARGV The special filehandle that iterates over command-
line filenames in @ARGV. Usually written as the null
filehandle in the angle operator "<>". Note that
currently "ARGV" only has its magical effect within
the "<>" operator; elsewhere it is just a plain
filehandle corresponding to the last file opened by
"<>". In particular, passing "\*ARGV" as a parameter
to a function that expects a filehandle may not
cause your function to automatically read the con-
tents of all the files in @ARGV.
$ARGV contains the name of the current file when reading
from <>.
@ARGV The array @ARGV contains the command-line arguments
intended for the script. $#ARGV is generally the
number of arguments minus one, because $ARGV[0] is
the first argument, not the program's command name
itself. See $0 for the command name.
ARGVOUT The special filehandle that points to the currently
open output file when doing edit-in-place processing
with -i. Useful when you have to do a lot of
inserting and don't want to keep modifying $_. See
perlrun for the -i switch.
@F The array @F contains the fields of each line read
in when autosplit mode is turned on. See perlrun
for the -a switch. This array is package-specific,
and must be declared or given a full package name if
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not in package main when running under "strict
'vars'".
@INC The array @INC contains the list of places that the
"do EXPR", "require", or "use" constructs look for
their library files. It initially consists of the
arguments to any -I command-line switches, followed
by the default Perl library, probably
/usr/local/lib/perl, followed by ".", to represent
the current directory. ("." will not be appended if
taint checks are enabled, either by "-T" or by
"-t".) If you need to modify this at runtime, you
should use the "use lib" pragma to get the machine-
dependent library properly loaded also:
use lib '/mypath/libdir/';
use SomeMod;
You can also insert hooks into the file inclusion
system by putting Perl code directly into @INC.
Those hooks may be subroutine references, array
references or blessed objects. See "require" in
perlfunc for details.
@_ Within a subroutine the array @_ contains the param-
eters passed to that subroutine. See perlsub.
%INC The hash %INC contains entries for each filename
included via the "do", "require", or "use" opera-
tors. The key is the filename you specified (with
module names converted to pathnames), and the value
is the location of the file found. The "require"
operator uses this hash to determine whether a par-
ticular file has already been included.
If the file was loaded via a hook (e.g. a subroutine
reference, see "require" in perlfunc for a descrip-
tion of these hooks), this hook is by default
inserted into %INC in place of a filename. Note,
however, that the hook may have set the %INC entry
by itself to provide some more specific info.
%ENV
$ENV{expr}
The hash %ENV contains your current environment.
Setting a value in "ENV" changes the environment for
any child processes you subsequently fork() off.
%SIG
$SIG{expr}
The hash %SIG contains signal handlers for signals.
For example:
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sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
Using a value of 'IGNORE' usually has the effect of
ignoring the signal, except for the "CHLD" signal.
See perlipc for more about this special case.
Here are some other examples:
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
Be sure not to use a bareword as the name of a sig-
nal handler, lest you inadvertently call it.
If your system has the sigaction() function then
signal handlers are installed using it. This means
you get reliable signal handling.
The default delivery policy of signals changed in
Perl 5.8.0 from immediate (also known as "unsafe")
to deferred, also known as "safe signals". See per-
lipc for more information.
Certain internal hooks can be also set using the
%SIG hash. The routine indicated by $SIG{__WARN__}
is called when a warning message is about to be
printed. The warning message is passed as the first
argument. The presence of a __WARN__ hook causes
the ordinary printing of warnings to STDERR to be
suppressed. You can use this to save warnings in a
variable, or turn warnings into fatal errors, like
this:
local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;
The routine indicated by $SIG{__DIE__} is called
when a fatal exception is about to be thrown. The
error message is passed as the first argument. When
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a __DIE__ hook routine returns, the exception pro-
cessing continues as it would have in the absence of
the hook, unless the hook routine itself exits via a
"goto", a loop exit, or a die(). The "__DIE__"
handler is explicitly disabled during the call, so
that you can die from a "__DIE__" handler. Simi-
larly for "__WARN__".
Due to an implementation glitch, the $SIG{__DIE__}
hook is called even inside an eval(). Do not use
this to rewrite a pending exception in $@, or as a
bizarre substitute for overriding
CORE::GLOBAL::die(). This strange action at a dis-
tance may be fixed in a future release so that
$SIG{__DIE__} is only called if your program is
about to exit, as was the original intent. Any
other use is deprecated.
"__DIE__"/"__WARN__" handlers are very special in
one respect: they may be called to report (probable)
errors found by the parser. In such a case the
parser may be in inconsistent state, so any attempt
to evaluate Perl code from such a handler will prob-
ably result in a segfault. This means that warnings
or errors that result from parsing Perl should be
used with extreme caution, like this:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give backtrace...
To see backtrace try starting Perl with -MCarp switch";
Here the first line will load Carp unless it is the
parser who called the handler. The second line will
print backtrace and die if Carp was available. The
third line will be executed only if Carp was not
available.
See "die" in perlfunc, "warn" in perlfunc, "eval" in
perlfunc, and warnings for additional information.
Error Indicators
The variables $@, $!, $^E, and $? contain information about
different types of error conditions that may appear during
execution of a Perl program. The variables are shown
ordered by the "distance" between the subsystem which
reported the error and the Perl process. They correspond to
errors detected by the Perl interpreter, C library, operat-
ing system, or an external program, respectively.
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To illustrate the differences between these variables, con-
sider the following Perl expression, which uses a single-
quoted string:
eval q{
open my $pipe, "/cdrom/install |" or die $!;
my @res = <$pipe>;
close $pipe or die "bad pipe: $?, $!";
};
After execution of this statement all 4 variables may have
been set.
$@ is set if the string to be "eval"-ed did not compile
(this may happen if "open" or "close" were imported with bad
prototypes), or if Perl code executed during evaluation
die()d . In these cases the value of $@ is the compile
error, or the argument to "die" (which will interpolate $!
and $?). (See also Fatal, though.)
When the eval() expression above is executed, open(),
"<PIPE>", and "close" are translated to calls in the C run-
time library and thence to the operating system kernel. $!
is set to the C library's "errno" if one of these calls
fails.
Under a few operating systems, $^E may contain a more ver-
bose error indicator, such as in this case, "CDROM tray not
closed." Systems that do not support extended error messages
leave $^E the same as $!.
Finally, $? may be set to non-0 value if the external pro-
gram /cdrom/install fails. The upper eight bits reflect
specific error conditions encountered by the program (the
program's exit() value). The lower eight bits reflect mode
of failure, like signal death and core dump information See
wait(2) for details. In contrast to $! and $^E, which are
set only if error condition is detected, the variable $? is
set on each "wait" or pipe "close", overwriting the old
value. This is more like $@, which on every eval() is
always set on failure and cleared on success.
For more details, see the individual descriptions at $@, $!,
$^E, and $?.
Technical Note on the Syntax of Variable Names
Variable names in Perl can have several formats. Usually,
they must begin with a letter or underscore, in which case
they can be arbitrarily long (up to an internal limit of 251
characters) and may contain letters, digits, underscores, or
the special sequence "::" or "'". In this case, the part
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before the last "::" or "'" is taken to be a package qualif-
ier; see perlmod.
Perl variable names may also be a sequence of digits or a
single punctuation or control character. These names are
all reserved for special uses by Perl; for example, the all-
digits names are used to hold data captured by backrefer-
ences after a regular expression match. Perl has a special
syntax for the single-control-character names: It under-
stands "^X" (caret "X") to mean the control-"X" character.
For example, the notation $^W (dollar-sign caret "W") is the
scalar variable whose name is the single character
control-"W". This is better than typing a literal
control-"W" into your program.
Finally, new in Perl 5.6, Perl variable names may be
alphanumeric strings that begin with control characters (or
better yet, a caret). These variables must be written in the
form "${^Foo}"; the braces are not optional. "${^Foo}"
denotes the scalar variable whose name is a control-"F" fol-
lowed by two "o"'s. These variables are reserved for future
special uses by Perl, except for the ones that begin with
"^_" (control-underscore or caret-underscore). No control-
character name that begins with "^_" will acquire a special
meaning in any future version of Perl; such names may there-
fore be used safely in programs. $^_ itself, however, is
reserved.
Perl identifiers that begin with digits, control characters,
or punctuation characters are exempt from the effects of the
"package" declaration and are always forced to be in package
"main"; they are also exempt from "strict 'vars'" errors. A
few other names are also exempt in these ways:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT _
SIG
In particular, the new special "${^_XYZ}" variables are
always taken to be in package "main", regardless of any
"package" declarations presently in scope.
Due to an unfortunate accident of Perl's implementation,
"use English" imposes a considerable performance penalty on
all regular expression matches in a program, regardless of
whether they occur in the scope of "use English". For that
reason, saying "use English" in libraries is strongly
discouraged. See the Devel::SawAmpersand module documenta-
tion from CPAN (
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http://www.cpan.org/modules/by-module/Devel/ ) for more
information.
Having to even think about the $^S variable in your excep-
tion handlers is simply wrong. $SIG{__DIE__} as currently
implemented invites grievous and difficult to track down
errors. Avoid it and use an "END{}" or CORE::GLOBAL::die
override instead.
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