You will want Test::Harness and Test::More. You probably already have them.

The easiest way to get started with those modules is to just examine the t/ directory of some CPAN module distributions. After that, take a look at the Test::More perldocs.

A recurrent bug can be swatted for good by turning the code fragment which triggers it into a short series of tests. Just compare the expected values to the actual ones for known input values. The test suite will then check for regressions each time it is run.

A Makefile.PL is a good thing to have, and not just for running the test suite, but you can also just call the runtests function of Test::Harness, $ perl -MTest::Harness -e'runtests @ARGV' t/*.t