Perl internal variables

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• $! - 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
• @ARGV - command line parameters
• @_ - parameters of the current function

Full list at perldoc perlvar