Well, if you've done some app in some other language, it might be educational to try redoing it in perl -- making sure that you add some enhancements to make the perl version a distinctly useful tool in its own right.

If you're approaching perl as your first programming language, then there are probably some other areas, involved with whatever OS you're using, that are new or unfamiliar to you as well, so you should consider what sort of app would allow you to learn/use/control your computing environment more effectively -- and write that app.