I know that almost no recommendation is going to be better than the previous one from Miyagawa, but anyway I think this module deserves a lot of recognition. It makes testing against servers almost painless, avoiding all the hassle around forking, searching for open ports in your system... We've used to test against HTTP servers, custom-made LDAP servers and even a FIPA server we had to integrate with. Great module!
This is an incredibly useful module to test TCP based server program.
All you have to do is to provide two callbacks: one is server to run the server side process, which could be perl code or a different program with exec(), and the other is the client code that tests the server process. Both would be called after the successful fork() and will automatically receive the empty port as a callback argument, which makes it so handy to use.
In Plack project we heavily use this module to run an HTTP server and then sends HTTP request to the forked server and back. Very handy.