Perl internal variables

$! - error of the last system call e.g. open file.
$/ - input record separator, slurp mode.
$_ - the default variable for many operations.
$0 - Name of the current program.
$1, $2, .. Regex match variables.
$] - version number of Perl
$a, $b - variables of sort.
@ARGV - command line parameters.
@_ - parameters of the current function.
@INC - list of directories to search for modules.
%INC - loaded modules and the path to them on the disk.
%ENV - environment variables
%SIG - signal handlers
Full list at perldoc perlvar