If I'm not sure how something works, I just write and run a tiny program to test that particular situation. With proper sectioning into subs, and liberal use of print and Data::Dumper, it's not too hard to verify if something is working as planned. For the most part, anyway - logic errors can be difficult to find, as well as the occasional little thing like using == instead of eq or = instead of ==. Depending on your input, a debugger may not identify that sort of problem.