There are several good reasons not to use mod_perl, most of which become even better reasons in a shared environment:

• it's a memory hog;
• it's very easy to "corrupt" other processes and scripts running under it;
• it gives access to web server internals;
• buggy perl code can take down a lot more than just the one buggy CGI;
• and so on.

Now, some of those can be worked around, but not all of them, and to do so does not provide, for the vast majority of "use cases", sufficient benefit to justify doing the work in the first place. If you really want mod_perl, then hire a cheap machine. From the nice people who host the machine I run CPANdeps on, for example.