To get the environment they're trying to build in mod_perl, all you have to do is set MaxRequestsPerChild to 1.
Except that mod_perl is still a single global environment for all Perl programs such that one customer's global changes could overwrite another customer's global changes.
If the "correct" solution to that is "why don't you just give each customer his own private server?", then mod_php still has clear superiority over mod_perl in the $4.95/month hosting market. I believe Clayton Christensen wrote authoritatively about the dangers of ignoring that.