Downloading a substantial CPAN package as-described and looking at its t directory really is probably the very-best source of practical real-world testing voodoo wisdom.

A reall-good test suite works up in stages, from low-level tests of every function of every object including exception cases, to substantially more-complex cases that build on the ones that came before.   You will be amazed at how many bugs you will find in your kitchen when you start looking systematically for them.   But, once they are gone, a test-suite can prove that they still are gone.   There is no substitute for that.