I'm not sure I really get your concept of a Perl MUSH, but what if you were to use something like Template Toolkit as the scripting language, and allow its [% PERL %] directive only after a user has attained a 'rank' that helps assure they are not going to intentionally abuse the system?

There is still the chance of accidental or unintentional abuse, or the patient malcontent willing to earn the rank needed to really bring down the system.