I spent a lot of time at the beginning of my project (bigtweet.com) writing some unit tests in the Perl Catalyst environment. I think that it was very worthwhile at the time while I was changing structure, implementing exception handling, etc. Now that the project has stablilized and I'm mostly working on incremental functionality, I've largely skipped writing new tests (due mostly to time constraints).

I still run "prove" before any new release though.

I've also experimented with Perl code coverage early on with good success. Again, time constraints prevent me from pursuing this further for now.

If you have the time, I would highly encourage writing some unit tests for the core functionality.