Depends on what you already know, doesn't it!

It also depends on the sort of job you want to do (if, for example, you want to work in scientific computing, BioPerl might be useful, but it's of little practical use elsewhere).

My current employer looks for people who know DBIx::Class, Catalyst and some Templatey thing, although if you're good enough and don't know them, then you're in with a good chance anyway, as it's not hard to pick up what's needed.

Probably the most important skill, and one that's hard to learn apart from on the job, is how to work with other peoples' code. I spend all day fixing, improving, and extending existing applications. You can pick this up to a certain extent by collaborating on open sores projects, but not to the same extent as you would on the job.

As to what I would study ... XS. In most jobs it's not needed that often, but when it is needed, few people know it. Also, I think it would be interesting.