As others indicate, there is little in the testing modules that relys on tests being in ".t" files. It's a feature for you to use, or not use. It's a very handy convention that will be understood widely, but as always, TIMTOWTDI.
A really wonderful book on testing, IMO, is Perl Testing: A Developer's Notebook. It's a compact and concise presentation of "here's how to do it" code examples, with supporting explanatory text to take you through the examples quickly. It covers a very wide range of testing scenarios, but it's organized so you don't have to read much more than just the section you're interrested in.
You can take a look at a sample chapter of the book on-line at the O'Reilly website. The book's Amazon rating is high, but there are a few who really didn't like it, so it's probably a good idea to take a look at the sample.