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One way to raise your expertise: expose yourself to
different languages, see what they do well, learn their
idioms, and apply that to Perl. Lisp is the best example
I can think of: if you get into Lisp, you'll learn all
kinds of cool tricks involving anonymous subs, closures,
lists, function application, and functional programming.
(You can get pretty much the same techniques out of
As a happy consequence, you'll also have a bigger toolchest of techniques to use. Some problems are hard with one tool, and trivial with another.
Another way, which works really well and shows results quickly, is to subject your code to relentless peer review. Having other people paw through your work, pointing out which bits are awkward, which are error-prone, which are hard to maintain, and sometimes which are elegant, is a tremendously valuable learning tool. If learning other languages expands your toolchest, code review sharpens the tools.
I can't emphasize enough how important code review is. I'd say the primary advantage to working on an existing Open Source project is that you'll be able to push your code on other experienced programmers for review.
As a happy consequence, you'll develop some extra humility.
Update: Clarified the Haskell/currying point: you'll miss Haskell's currying when you go back to Perl. (IIRC, Perl 6 will support curried functions, so maybe it's not so big a deal.)
Update 2: So you can get curried functions in Perl, albiet with a bit more trouble than in Haskell. Thanks, educated_foo!