This is a fine book, but the passage of time has rendered some parts of it less relevant.
As an introduction to object oriented programming, and how to do it in Perl, this is one of the best sources you could learn from. Neither The Alpaca nor The Camel do such a good job.
And it's not just objects that are well covered. You'll also find lucid explanations of closures, type globs, the symbol table and tied variables, all of which can be considered advanced Perl.
Elsewhere, though, the material has not aged so well, superseded by the author's own Perl Best Practices, where some of the recommendations have been reversed, or improved upon ('inside out' classes, for example, as implemented in Class::Std, is a superior development of the flyweight approach mentioned in this book). Some of the material, which concentrates on CPAN modules, and the experimental pseudohashes is not so useful in the light of this - the latter are on course to be removed in Perl 5.10. The sections on building objects using references to things other than hashes (e.g. arrays, regular expressions and subroutines) is clever, but this reader was unconvinced of their utility.
There's also coverage of generics, although in Perl this is not much like generics in C# or Java, basically passing around Perl code as uninterpolated text strings and then evaling it inside a subroutine, where any lexical values are interpolated.
Finally, there are chapters on multimethods (no more elegant or manageable in Perl than other languages that support this feature, alas) and persistence.
The principles discussed remain relevant, and the book is a pleasure to read. However, if you already familiar with OOP and just want to get going as fast as possible, the relevant chapters of Intermediate Perl and Perl Best Practices might be better places to look.