If they know C++, they can probably think for themselves and learn another language on their own. 1-day isn't enough time for spoonfeeding.
Given they'll be able to pick up the syntax (hopefully they will). I'd show off features and let them learn some on their own. Hashes since they won't be used to them, auto-vivification, all the goodies on CPAN for really obscure stuff that would be incredibly hard in C, the cleanliness of Tk (if they know Java for example, they'll probably hate Swing), functional programming to attempt to blow their minds, etc. They'll be coming back for more if you do it right...
Also, I think you need to give at least one example of something that is incredibly hard to do in most languages, done in one line of Perl code. Open with it. Something like "who is winning the most democratic primaries by scraping CNN" or stock prices or something like that.