Bottom line:

If you run mod_perl OR mod_php on the same config that your static pages are served from, you will have apache processes that are larger (and therefor you can run fewer on the same hardware/memory) and that can effect your static page serve rate. If static serve rate needs to be balls to the walls fast, you have two choices. 1) Dual install apache and limit how many of the apache processes are running mod_perl (only the amount needed for the mod_perl page serve, in your case few) and the static site (only serves static pages -- highly tuned config). 2) run the static content off another server.