This is a difficult question. For one thing, what purpose would it serve to motivate yourself to use a language you don't need for work? If you don't already find yourself motivated to play with it, then it's just not fun for you right now.

One suggestion for getting interested might be to try to find some program that the world seems to be missing, that you feel you could write in Perl. The idea is to have fun - so plowing through a book probably won't do it. It seems more fun to just go ahead and try stuff, and use the books for guidance when you get stuck. But everyone has a different style of doing things...