If you've got most of the O'Reilly Perl books you've got a year's supply of reading available to you, most of which is top notch. But branching out from there to other publishers, you'll find that Modern Perl is well thought-of, as is Higher Order Perl. There's Sam Treagar's book on creating CPAN distributions, there's Lincoln Stein's book on Network Programming with Perl (an oldie but goodie). Conway's book on Object Oriented techniques with Perl is getting really dated, especially since Moose is becoming so popular. But nevertheless, it's a great book because it teaches the nuts and bolts that you don't get exposed to directly when using frameworks like Moose.

I've also found that studying other languages has helped me to improve my Perl. ...Perl wasn't my first language, but having spent so much time with it, I'm discovering that now examining other languages teaches me a lot.

Then of course there's plain old day to day use. The more you use it, the more familiar you will become, and the more Perl's expressiveness will become an extension of your thought processes.

Last, but not least, get involved with your local Perl community. Attend lectures and presentations. Watch the YAPC presentations on youtube. There's a lot to be learned by being around others who are using Perl every day.