in reply to Perl, PHP mod_perl
First you should determine whether mod_perl will actually help. mod_perl is helpful when you've got a LOT of requests coming in for a resource, and each of those requests result in a process being spawned, and this process start/stop cycle can take up a lot of resources. If this is the case, mod_perl can help (you're having resource issues because you have too many requests coming in).
However, if your perl script is taking up resources because it's doing something resource-intensive, likely mod_perl won't help. For example, if it's only accessed occasionally and if it's taking an hour to run and pegging the cpu/disk/memory/whatever because it's processing a huge file, mod_perl won't help.