Well, first of all Higher-Order Perl is really good, but it's about functional programming, not objects.
Perl Best Practices is an excellent book, but it has limitations, and one of them is that Conway recommends using his own Class::Std module to do inside-out objects, and the general consensus seems to be (1) that if you're going to do inside-out objects, Object::InsideOut and Class::InsideOut are better ways to do it (2) and anyway, using "Moose" based objects is a better way to go.
This illustrates what is probably the major difference between the Java and Perl world: There's rarely one standard way of doing anything with Perl. Starting as someone who feels comfortable with objects in another language, I would guess that the most interesting thing about Conway's now slightly dated Object Oriented Perl is watching him gradually develop different ways of adding OOP features you've been taking for granted.
I like the basic blessed-hash style of Perl OOP myself, but you need to understand that it's encapsulation is really weak, and that while method-inheritance works, there's typically no data-inheritance. There are also some rather perlish tricks in wide use, such as automatically generating accessors using an AUTOLOAD routine.
As for what you can read on the subject, don't neglect the on-line documentation that comes with Perl: perldoc. Note the "Tutorials" section at the top. If you're rusty on Perl's references and data structures, read the first two: perldoc perlreftut and perldoc perldsc. A little down the list, you'll see multiple OOP tutorials. These are largely about simple href-based objects, though there are some serious oddities in there, such as Tom Christiansen's scheme for closure-based objects down at the bottom of: perldoc perltoot
If you're interested in some of the newer ways people do things, you might want to start with Moose which is supposed to be the closest you can get to perl6 objects while still writing perl5 code. By the way: ignore the word "postmodern" there, it's a silly joke that doesn't make any sense.
If you're interested in inside-out objects (which have bullet proof encapsulation, but are perhaps a little annoying to debug-- you can't just use Data::Dumper on the object to get it's status), I'd suggest starting with this perl5 wiki page.
