Even under mod_perl, there are probably multiple server kids serving requests, so there's little guarantee you'll reach the same process as before. You're right that you shouldn't rely on this... but fortunately, it's really hard (but not impossible) to turn a stringified reference back into a real reference.

There are times in which a similar technique works, though. Continuations can work really well for web programming, though Perl 5 doesn't really support them natively. You'll probably be sorry for asking what they are unless you've used Scheme.