how about the perlito5 compiler?

it is also available as a plugin for the Farabi web-based Perl IDE

Perlito5 - http://perlcabal.org/~fglock/perlito5.html

Farabi - http://search.cpan.org/dist/Farabi

You would be looking at creating an interpreter that works with code that looks like Perl. This is not a small task to do properly.

Since the goal is to teach programming concepts, why not take a look at Scratch, a programming language for kids? It was designed at MIT and for ages 6+.

... I just want to limit the students' ability to shoot themselves (and each other) in the foot ...

A properly structured and maintained System Administrator/User system should prevent students shooting others in the foot, and shooting yourself in the foot is a learning experience!

... and also limit their access to cheat by taking liberties with namespaces and object innards.

But "cheating" and "taking liberties" (also known as "playing") are part of the fun of learning — and often a royal road to enlightenment. All too soon will your students enter an academic or commercial environment (whether or not they seek to become professional programmers or engineers) in which Someone will constantly be watching over their shoulders to make sure they don't "take liberties." For now, let them have a chance to enjoy learning.

So don't bother. Just set up effective user firewalls, give them Perl, let them go nuts. You may be quite pleasantly surprised!

(I'm assuming the kids are about 10 years and up. Younger than that, something like jfroebe's suggestion may be more appropriate than Perl.)

A properly structured and maintained System Administrator/User system should prevent students shooting others in the foot, and shooting yourself in the foot is a learning experience!

Oh, I agree. Securing the system from a bunch of kids is relatively easy (but I've learned to never underestimate the tenacity of a fifth grader who wants to beat his friends in some kind of real or imagined competition). And, yes, shooting oneself in the foot is educational, although often painful. The main point here is to provide an environment with a limited number of pitfalls that will lead to needless frustration.

For example, nothing I teach them will ever be solved with shell commands, but I can pretty much guarantee someone would accidentally type print `which cat is your favorite?` and be intensely confused by what /bin/cat has to do with anything. To my mind, it would be a much less frustrating experience to get a syntax error (or at least an I'm sorry, Dave. I'm afraid I can't do that.) pointing to that line. I fully believe a little frustration can stimulate learning, but a lot of frustration combined with pretty much no clue what's going on, can lead to rather significant detachment. Yep, that's life, but fifth-graders are still developing a lot of those personal skills, and even among adults, getting thrown into the deep end before even learning the doggy paddle (be that figuratively or literally) is often counter-productive. Of course this is more developmental psychology than Perl, but hopefully it helps illuminate my motivations for wanting to throttle the kids' frustration level.

But "cheating" and "taking liberties" (also known as "playing") are part of the fun of learning — and often a royal road to enlightenment.

Also agreed (emphatically!). While I of course try to project positive values re: honesty and cheating, I also don't step in to stop it unless there's a very good reason to do so. Game playing is a relatively safe (and efficient!) way to experience both sides of the social consequences of dishonesty, the rush of thinking you got away with it, and the crushing embarrassment and anger of getting caught (or finding out you've been bamboozled). In the context of what I'm currently proposing, there will be ways to "cheat"--some of them deliberate on my part. I just want to be able to limit what they can and can't get away with.

Thanks for the insightful comments! I agree with them. If any of this seemed contrary, it's just me trying to elaborate a bit on some of the deeper motivations behind this project.

Safe (and Opcode) looks great! I had no idea core Perl had that ability. That will easily facilitate much of what I need to do, and I can even set up different capabilities on the fly. Nice.