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 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.

You asked for personal experience and opinions, so my post is purely written from that perspective though I recognize different things work for different people. I would say that 90% of the code that I write I never use, an additional 7% is toy programming, leaving about 3% being used practically.

I program because it is a nice balance between the freeform creative side and the very rigid logical side of my brain. I spend vast amounts of time at the Monastery and #perl on IRC solving other people's problems. I try and find interesting problems to work on for no other reason than to learn and grow.

I believe the previous paragraph answers both your first question and your second. Your third question to me is highly dependent on your situation. Currently, I don't program by trade though writing code is a tool I use in my job. That means what is essential to me now is unlikely to be essential for me in a different environment.

To me it boils down to repeatedly using techniques until you no longer have to think about them. Once you assimilate them, you code them naturally when they are appropriate. I expressed this in several Meditations. A couple of the most applicable are Value of "RE"-coding for the newbie and Necessity is the mother of invention. Force yourself out of your comfort zone gradually learning new things in an ever expanding circle of knowledge.