Another way to get your feet wet might be to look into the Phalanx project. It's a project to develop better testing suites for core Perl modules. The benefit to you, as you are thinking about writing a module, is that you can work with the maintainer of a top 100 Perl module and pick their brain a bit. You can see how they have done things, and get feedback on how they might do things differently now. Plus you'll get some testing experience and contribute to a core Perl module.

Although it's slightly off-topic, I can't believe petdance didn't mention it :)