Perl.com: Making Sense of Subroutines:

Subroutines are the building blocks of programs. Yet, too many programmers use them ineffectively, whether not making enough of them, naming them poorly, combining too many concepts into one, or any of a dozen other problems. Used properly, they can make your programs shorter, faster, and more maintainable. Rob Kinyon shows the benefits and advanced uses that come from revisiting the basics of subroutines in Perl.

(Via Perl.com.)