Let me add one thing that no one's mentioned yet -- Test Driven Development.

At $work[-2] I developed a web application called File Exchange Server (FES) partially using TDD, and it opened my eyes to the whole process of a) starting with a known good system, b) adding a feature or bug fix and c) confirming that everything's still fine using your tests. Wow -- I change something over here (waving my hand over to stage left) and something over there (waving to the balcony) breaks! Cool! Now, why is that? Go find out.

My colleague Mike Stok turned me on to Log::Log4perl, a fantastically useful module for logging that allows you to turn up or turn down the logging level externally to whatever you're testing, with module granularity. Messages can go to a log file, they can be sent out as E-Mail messages, sent to a pipe, anything. Really, really useful.

As for what to build, try to build something you'd be interested in using, perhaps using wheels that exist already, perhaps using wheels that you write yourself. One thing that I want to do is sample my 200+ vinyl albums into MP3 files and make a jukebox out of the resultant recordings. I could use my wife's installation of iTunes, but it's a thought -- if I had the spare cycles.

A final word of advice -- keep your career in mind -- if software development is something you're interested in doing as a career, then when it comes to interviewing or even just talking about an opportunity, it's great to have some 'war stories' that you can roll out that embody your passion. That's your chance to talk about some of the cool projects that you completed, whether you used Perl or not.

Most of all, have fun!