I'm not sure what I would call essential, but I know what's high on my list right now. Following Tom Christiansen's observation (quoted by tilly here) that:

[a] programmer who hasn't been exposed to all four of the imperative, functional, objective, and logical programming styles has one or more conceptual blindspots. It's like knowing how to boil but not fry. Programming is not a skill one develops in five easy lessons.

I decided to broaden my horizons by (re)learning Scheme, which I haven't used at all since my undergraduate days. Maybe Prolog next, on the "logical programming" front (though I'm open to suggestions). Or Haskell. Or Eiffel. Sheesh. OK, what the heck, SNOBOL too.

I also want to spend some quality time with the Perl internals, but I confess that waiting for "the other shoe to drop" with Perl 6 saps some of the motivation.

Friedel's book on regular expressions has been on my to-read list for a while (but if I heed Mugatu's reply to my last meditation, maybe it should stay there for a bit longer).

I must say, however, and I mean this very sincerely, reading PM is for me the most effective way to sharpen my Perl and my programming. Hardly a day goes by that I don't learn something extremely useful here. So much of programming seems to be a matter of small little tricks and bits and pieces of knowledge (the name of a built-in function or of a CPAN module or of a Unix utility) that do not fit into some grand over-arching scheme. Bags of tricks, basically. Of course, bags of tricks don't lend themselves as the basis for university classes and best-selling books (though O'Reilly's hacks series is challenging that); next to face-to-face exchange with co-workers and other programmers, a place like PM is the best medium for this sort of knowledge, IMO.