Methinks that this might have a useful application in some sphere ( perhaps a mutation of this idea? ) but the farthest it can go is a module and a tutorial explaining the idea to programmers. I do not beleive this is worth implementing into Perl itself, I also don't believe in preventing programmers from making mistakes - if I didn't make mistakes I'd go crazy.
This could probably be implemented as an object that's like a cache for data - it encrypts it on the way in, and then you have to call a decrypt routine on the way out. This will prevent data turning up in error messages.
I think what pjf wants is a system where the data will have to be explicitly used, instead of restricting the use (make the programmer completely aware of what he(she?) is doing).

(Yes, I do realize that I denounced it and then provided a possible implementation... perhaps I am crazy already)