At Re^2: Mod Perl Usage on Virtual hosted domains, I asked why mod_php can be offered on shared solutions, while there is no way mod_perl could, to which crazyinsomniac replied that there is no way in perl to prevent a program from modifying `import' among other things, so that you can't share a perl process among different users.
And I just had the proverbial lightbulb go on above my head (though I suspect it will still prove to be a shot in the dark): wouldn't Safe let us do just that? If each user's code is loaded into a compartment, it shouldn't be possible for the different compartments to interact. In fact, we might get to simplify the process of mod_perlifying existing scripts as they believe they're running in main:: that way.
The idea seems so obvious in retrospect that I'm wondering if I'm really the first this occurs to or there is actually a totally obvious pitfall that I'm missing that noone ever bothered because of.
I've been pondering this problem on and off for a while. PHP has spread through the shared hosting world like wildfire, as is easily evidenced by the fact that the simple web messageboard domain is dominated by PHP nowadays. The feasibility of being offered in shared hosting environments would be a great boon to mod_perl.
Makeshifts last the longest.