I acknowledge that BrowserUK has a point on the numbers, dots and percentages scroll, but I disagree in that this should prevent one from writing tests first. I usually write one (or more) test file(s) per sub, which again makes it fairly simple to narrow down on specific code, and using diag and the more verbose functions of Test::More et al. should render it obvious what went wrong in the event of failure.

As for pitfalls, I think of easy of change. Having both a test suite and code (as opposed to just code) means there is more to change. For smaller scripts where you feel very confident, having to rewrite the tests and the code can be burdensome. Then again, that may be a good thing as it forces you to think while letting you change one section at the time and making sure that it all still works as intended. In overall I don't this this should prevent you from writing (at least some or most of the) tests first.