My trilogy:

• The Llama Book
• The Camel Book
• The Black Leopard Book

The llama book is my first Perl book, and it helped me step into the world of Perl. My first impression is, this book is a basic sketch of the funky Perl world ("look at the way they talk!" - the style of writing, the funny names of variables, the speak-like programming structures, yet how (un)friendly Perl is) :-P

For the Camel book, as someone figured out before, a little bit like a print-out version of perldoc, but hey, I simply love the way Larry(and the co-authors) write(I'd like to use the word "sermon"), "Part bible, part encyclopedia, and part almanac, this is the essential book on Perl.", as the back cover reads. Also, you will find some cultural background and some "reading material" from the book.

The Black Leopard, i.e., Advanced Perl Programming, is the book showed me more parts of the Perl world. I got a taste of "hacking" right after finishing reading the first chapter ("Advanced Techniques"). The rest part of the book unleashes some secret corners, stuff the readers' package of hacking. I'm sure it is kind of a must-read (from apotheon's node) for a beginning Perl hacker.

There's also some other books, among them, Higher Order Perl by Mark Jason Dominus, helped me a lot. I turned to Perl after wet my feet with water from Scheme/Common Lisp, this book showed me how to think functional in Perl, and it was an enjoyable reading.