If you're new to Unit Testing, I recommend reading one of the "walkthrough" books. You'll come away with a some new tricks in your bag, even if the target language isn't Perl.

Kent Beck's Test Driven Design: By Example is good, as is Newkirk and Martin's Extreme Programming in Practice. (I thought this was a one-read book, but I'm now going through it a second time.) Also worthwhile is a series Ron Jeffries is working up on Test Driven Development in C#.

Unlike many books that show only the finished product, these works show false starts and misteps, and how the authors recovered from them. They also all use JUnit (or the equivalent), but the ideas you'll pick up are readily transferable to Perl.