I agree with the suggestions above. Hashes, especially, change the way you think about data.

Regex in Perl is pure gold too, and could take months to teach if you went into any amount of detail.

One thing I'd do is teach how to get information in Perl. Not just CPAN (though that's a great suggestion cuz there's so much to do), but perldoc, and just how simple it is to formulate a quickie test program. I'd think that perltut would be a decent place to start.

For me, I use Perl most often, not to build a massive production piece of software, but just to automate the environment and the data generated by the legacy systems I'm consigned to maintain. So getting text/arguments/pipes/sockets/data into my program and doing something quick with it, etc... is very useful to me. Perl's so nice in that way. I also love how Perl handles References--the hash of hash (of etc.) is a beautiful thing.