I know why it is a problem in Perl, and I think I have a sense of why there is no good fix for the problem, but I cannot articulate it very well.
For those who don't know, if you have a my around a sub and the my is executed multiple times, you then wind up with multiple copies of the my variable (one per invocation), and there are deep naming problems making it impossible to give a clean answer as to which copy of the my goes with the globally named sub.
Perl doesn't answer it, it just gives you a "Cannot stay shared" warning and proceeds to not do what you might expect. If you use my variables like globals in your script in mod_perl, you will hit this because mod_perl executes your script on each call. Execute it twice and now all of those global mys are not shared properly and you have serious voodoo.
This is a very good reason to be in the habit of using vars when you really are thinking of globals. (Based on first principles I would want to check in 5.6 whether carelessness with "our" could also hit this problem. I honestly don't know the answer.)
/tell tilly if anyone needs clarification on this.