the one I actually found most useful while learning was 'Perl 5 Interactive Course'. Written by John Orwant (once of the porters, IIRC), and pretty fine at introducing both some programming notions and Perl (since I was new to both). Since then, the Cookbook, OO-Perl, and the Camel book have been indispensible. I learned a bit from reading/ applying Learning Perl, but it came together after the 'Course' book.