philcrow mentioned Damian's Book. In addition, you may want to take a look at The Alpaca book by Randal Schwartz. It's a bit newer and sort of eases you in to OO Perl. At least it did in the first edition when it was called Learning Perl Objects, References, and Modules.

After those two, if you like C++ and Java OO for their strong data encapsulation, Damian has some things to offer on what's recently become en vogue for OO Perl: Inside-Out Objects. Some introduction to this is given in his Best Practices book. He's also got a module or two to help you build these kind of classes.

Happy Hacking!