Although not written for a presentation, I find the first chapter of the Learning Perl book by merlyn a great starting point. If you are in a hurry, you can find it online here.(Thanks, Merlyn!) And if you have a day before the class, go buy the book and encourage your students to buy one. I use it as the text of my own Perl course and find it very clear and easy for students to understand. Heck, that's how I learned Perl.