Thank you for posting this question. ++
This hits home for me since my wife and I are working on a way to integrate programming into classrooms for a variety of subjects (specifically art and current events). The age and environments are different (grammar to middle school with the first program running at an after school program and the second possibly at a regional grammar school in Nicaragua.) But we still have to do similar things like lesson plans with either one big class project or smaller group/individual projects. Most importantly we have the common challenge of connecting with the kids, designing a program that's relevant to them.
The challenge of keeping their interest is tough but I do think there is value in saying "Hey, you guys like World of Warcraft, Facebook or have a favorite iPhone app? Well each are created using these languages and this is what they look like..." They wont learn them then and there but for the ones who will eventually choose this as a calling, lifting OZ's curtain can have a big impact.
What I've found is that Etoys seems to be more appropriate for kids up to 12. Scratch (which looks like you've settled on), Processing or even Squeak I think are pretty right on for high school students IMO. I LOVE ELISHEVA's suggestions and don't see why you couldn't break the class into smaller group projects that give them an opportunity to experiment with each of these tools and methods.
Best of luck and please keep us posted on your experience teaching the class.
P.S. I second your desire to see something like Scratch or Processing written in Perl.
"...the adversities born of well-placed thoughts should be considered mercies rather than misfortunes." — Don Quixote