Special Variables
$_
$_ is the "it" variable.
It's often the default parm that built-in functions use,
or return into.
while ( <> ) { # Read a line into $_
print lc; # print lc($_)
}
This is the same as
while ( $it = <> ) {
print lc($it);
}
$0
$0 contains the name of the program being run, as given to the shell. If the program was run directly through the Perl interpreter, $0 contains the file name.
$ cat file.pl
#!/usr/bin/perl
print $0, "\n";
$ ./file.pl
file.pl
$ perl file.pl
file.pl
$ perl ./file.pl
./file.pl
$ cat file.pl | perl
-
$0 is what C programmers would expect to find as the first element of the argv array.
@ARGV
@ARGV contains the arguments given to the program, as ordered by the shell.
$ perl -e 'print join( ", ", @ARGV), "\n"' 1 2 3
1, 2, 3
$ perl -e 'print join( ", ", @ARGV), "\n"' 1 "2 3" 4
1, 2 3, 4
C programmers may be confused, since $ARGV[0] is really their argv[1]. Don't let this mistake happen to you!
@INC
@INC contains all the paths that Perl will search to find a module.
Perl programmers used to append or prepend to @INC to add a library path; these days, use lib is used instead. The following are mostly equivalent:
BEGIN { unshift @INC, "local/lib" };
use lib "local/lib";
%ENV
%ENV contains a copy of the current environment. It is the environment that will be given to any subshell created by Perl.
This is significant in taint mode, as %ENV will have entries that can alter the shell's behavior. For that reason, perlsec recommends the following code be used prior to executing a command in taint mode:
$ENV{'PATH'} = '/bin:/usr/bin'; # change to your real path
delete @ENV{'IFS', 'CDPATH', 'ENV', 'BASH_ENV'};
%SIG
Perl has rich signal handling capabilities; using the %SIG variable, you can make any subroutine run when a signal is sent to the running process.
This is especially useful if you have a long-running process, and would like to reload configuration files by sending a signal (usually SIGHU/code) instead of having to start and stop the process.
You can also change the behavior of die and warn by assigning to $SIG{__DIE__} and $SIG{__WARN__}, respectively.
<>
The "diamond operator", <> is used when a program is expecting input, but isn't concerned how it arrives.
If the program receives any arguments, they are taken to be file names and their contents are sent through <>. Otherwise, standard input (STDIN) is used.
<> is especially useful for filter programs.
<DATA> and __DATA__
If a program contains the magic token __DATA__ on a line by itself, anything following it will be available to the program through the magic <DATA> filehandle.
This is useful if you want to include data with your program, but want to keep it separated from the main program logic.
$!
When running any command that uses the system, $! will be true if the command returned a non-true status, or otherwise could not be run. $! will contain the error.
$@
If using eval, $@ contains the syntax error that the eval threw, if any.