Take a look at "The Perl Cookbook" - almost every recipe in there would make a good class topic.

Or - browse topics on this site for a few days - especially "Seekers of Perl Wisdom" - people here come up with some real doozies!

In general, any problem that involves lots of file manipulation and/or text processing lends itself to a good perl solution.

If you're presenting to an audience that wants to know "what's it good for?" or "why should I care?", consider this: Unix has a large toolbox of text processing utilities: sed, grep, awk, etc - just to name a few. Perl is like all of these rolled up into one nice package. The syntax is similar to C/C++, so most programmers can learn it pretty fast. It's not called the "swiss army knife of programming languages" for nothing!

Another big selling point: CPAN. They say 90% of any perl project has already been written by sombody, somewhere - and it's all there for free.