Chapter 5. Variables

Variables won't. Constants aren't.
Osborn's Law

Compared to most mainstream languages, Perl has an embarrassingly rich variety of built-in variables. The largest group of these are the global punctuation variables$_, $/, $|, @_, @+, %!, %^Hwhich control a wide range of fundamental program behaviours, and which are largely responsible for Perl's unwarranted reputation as "executable line-noise". Other standard variables have more obvious names@ARGV, %SIG, ${^TAINT}but are still global in their scope, and in their effects as well.

Perl also provides self-declaring package variables. These will silently spring into existence the first time they're referred to, helpfully converting typos into valid, but incorrect, code.

This chapter presents a series of coding practices that can minimize the problems associated with Perl's sometimes over-helpful built-in variables. It also offers some techniques for making the most efficient use of variables you create yourself.