The Test::Simple distribution comes with a pretty good tutorial on testing.

Another very handy tool is Devel::Cover. It can run your test suite with debugger hooks in place to log which lines of code have been executed and which branches have been followed. It can then produce fancy HTML reports that tell you what percentage of your code is exercised by the test suite and even colour codes the lines of source. Drill down into some of the links on this page for an example.