Configure the new perl(s) with -Dversiononly. That way, you get not only a versioned perl executable (e.g. perl5.8.6), but a perldoc5.8.6, perlbug5.8.6, etc.

Then you need to choose between installing all modules for each version, or sharing (to the extent possible by binary compatibility).

I'm not sure where the maintainence nightmare comes into it; either you need to keep old perls around, or you don't. If you have old scripts depending on an old version of perl, you wouldn't typically touch that version at all, would you?