I think one of the most awesome things about doing open source/free software work is working with others toward a common goal. Of course, when you hold the reins of a projects you can't do everything to please everyone. So the simplest course of action is to encourage people to write failing tests for the functionality they would like to see.
This is surprisingly effective for getting regular, active contributors. Of course, once the test is written, and the contributor is confident it tests what they want it to test, it is a simple hop to peering into the code itself and making their own test pass. And usually all that takes is guiding the new found contributor in the right direction: "Oh, your testing for stuff in Flarg.pm, you know, you could probably add that easily if you do X".
I think that near-instant gratification is important and it makes herding the cats that much easier. It makes my life that much easier and I am sure I make other people's lives easier when I participate in the same fashion.
That said, Perl has a great testing culture developed that makes it very easy to cast the "Write me a test" net, far and wide. Plunk down a new t/foo.t, and fire it off with prove -l. No need to spin up a gigantic harness to make the magic happen.
mst is (in)famous for getting work out of people by suggesting projects and ideas to others. Even going as far as private messaging me on IRC about an orphaned module that could use some POEx::SessionInstantiation lovin'. And his method for encouraging contributors is equally as valid and important as well.
In the grand scheme of things, we are all trying to make software development easier. Perl, CPAN, its culture of testing, the community at large-- It all just makes it that much better.