Test::Unit will be immediately familiar if you've used JUnit. It's highly object oriented and works well if you use it as it's intended to be used.

Test and Test::Harness have the advantage of being in the core distribution. They require slightly more initialization on your part, but they're not too difficult to use.

jlp and I discussed this the other day, and he ended up writing 50 lines of code that did all of the testing we needed. It follows clemburg's Test::Unit framework, but it's limited to the bare bones of what we need. If you just want simple tests, it's very easy to use.