I have no doubt that you understand why -l works for you, but just to provide a counterpoint, I don't use -l for my own distributions, as I like putting the main .pm file in the base directory of the unpacked distribution, for the simple matter that it makes it a tad easier to pull into an editor.

There are also distributions that munge the .pm significantly on its flight from ./lib (or .) to ./blib, or even generate it ex nihilo. In these circumstances, -l doesn't produce the same results as -b.

That said, I understand why you like -l. It means you can edit the source .pm file, and run the prove test immediately, without having to run make (or ./Build) in between. There's nothing worse than making a change to the module, and then spending considerable time wondering why it didn't fix the test suite :)