My trilogy would be:
- The Llama book
- This is the "HOWTO" that beginners need in addition to the manpages. The Camel isn't on my list because it's, to a very large degree, just a restatement of the manpages.
- Effective Perl Programming
- The first edition from back at the dawn of time is still a damned good book for beginners. I'm very much looking forward to reading the new second edition.
- Object-Oriented Perl
- The Damian Book is also ancient, but still very good. Sure, it doesn't cover new stuff like Moose, but it will give you an excellent grounding in the basics.
And I'd also like to mention Mastering Regular Expressions. I've only read the first edition, but that was excellent, and an awful lot of what I got out of it is just as useful in other languages as it is in perl.