The book that follows me wherever I code in Perl is O'Reilly's Perl in a Nutshell, an excellent reference text that's smallish and tells me everything I need to know. The book that I checked out from the library once and I plan to buy so that I can put it under my pillow when I sleep is Higher-Order Perl: Transforming Programs with Programs by Mark Jason Dominus. I can honestly say that it took my Perl code to the next level.