Great suggestions so far, so I want to throw in one of the most important books for my own personal Perl growth, Learning Perl Objects, References and Modules by Randal Schwartz and Tom Phoenix.

I also can't say enough about what Your Mother said regarding the reading of CPAN distribution test files.

Although much of the time the author will write code in ways that kind of defy the logic of standard practices (ie./eg. I write a lot of my tests to be short to the point that brevity is above all else), it's a fantastic way to identify what the author meant by the use of the software the tests are running against.

You can compare test elements to the code within the actual modules, and to further knowledge, practice by writing additional tests where you feel the original authors may not have thought about certain edge cases. If desired, you could even have the author add these new tests.