Maybe I'm a bit out of touch with what 8-year-olds would be capable of or interested in, but it's probably a good idea to avoid Perl. I don't think there's any way an 8-year-old can learn anything interesting in or about Perl in 20 minutes.

If you want to give them a good starting point for learning to program, show them Logo and teach them some of the basics. In 20 minutes, you can probably introduce them to instruction statements and looping, and get them to write a simple program to draw a square using turtle graphics. It's not much, but it lets them actually try to do something themselves.