I'm not sure about mod_perl, because it will have to be rewritten for Perl6 (and/or Parrot) anyway. The issue to keep in mind here is that mod_perl has only one shared interpreter and namespace for one Apache process while it's desireable for shared hosting to separate them.

I'm not sure if the approach used by mod_perl or mod_php is sensible enough anyway, because it doesn't allow finer-grained control and still has the possibility of leaking/accessing information of other contexts. There are other, well-supported frameworks like FastCGI, which keep every environment confined to its own process while avoiding the performance penalty of starting a process new for every page request.