Conway's "Object-Oriented Perl" is indeed a pretty good book, and doesn't deserve to be dismissed as dated. It was written before the current fad for inside-out objects, but if you're interested in understanding the more traditional hashref-based objects, it's a good place to start.

A few caveats:

• It praises pseudo-hashes, a feature that was immediately regarded as a mistake and was quickly deprecated and removed.
• Throughout, it uses an example that's arguably better suited toward a relational database approach -- it may explain object design very well, but it's not necessarily an example of good design.