Object Oriented Perl and Elements of Programming in Perl are the only non-animal perl books I've ever wanted or needed. Since I don't have a CS degree or any formal instruction in programming, I found that these books really filled in some of the major conceptual gaps in my knowledge, as well as providing me with some handy tools to hang on my belt. The O'Reilly books that helped do this too are Mastering Algorithms in Perl and Perl for Systems Administration

I think that one of the best features of Perl is the presense of definitive texts, like the Camel and Cookbook. To me, Object Oriented Perl is the OOPerl book. If it were an O'Reilly book, I might be tempted to put a Two-Humped Camel on the cover. Or at the very least an alpaca...