I don't know. But I am interested.

What sort of loads does your mod_perl application do? Is it a chat system, a blog, a heavy database query system, a simple web form, a game server? How many people connect to your system - requests per second and concurrent users? How many webservers are simultaneously serving this application (is it on a single box)? Is the database on the same box? Is the database mysql, postgres, a nosql, or some other? Do you have any caching layers (memcache, kyoto, file based)?

I'm interested in what aspects of your application have led you to async operations as the solution, even if it is just "to scratch that itch." I think you'll get more comprehensive feedback if you fill in a few more blanks.