It would be quite easy for someone to accidentally code something that will use all available memory or consume all CPU in a tight loop. There are ways to deal with this (rlimits), but Safe will not prevent it.
in reply to mod_perl and shared environments don't mix - do they?
Perhaps more to the point, who is the market for this? I wouldn't pay a nickel for a hosting environment that required my code to run under Safe, or one that wouldn't let me modify httpd.conf and startup.pl.
Incidentally, PHP doesn't sound very safe either. You could do the things I mentioned above with it too. Can someone who knows more about it explain why PHP is considered okay? Does it run setuid or something? Or maybe it's just that it uses less memory.