Write lots of it and use any spare time and breaks browsing sites like this one to see how other people approach problems.

Update/More
I'm an IT contractor, and I do short term contracts in a number of different languages. The good side of this is that I have a wide ranging skillset, keeping me employed, the bad side is that I never really get to be a "guru" in any one particular area.

For Perl, I've found PerlMonks is a useful way of keeping my hand in, and also seeing that other people solve problems in a different way from the way I would do it introduces me to novel concepts and the flexibility of the language. A skill left unused atrophies, so sites like this one are a great way to stay fresh.