I expect that since mod_perl is a binary module it needs to be installed once per perl interpreter version. Certainly perl5.8.8 would not look in the 5.8.0 library directory by default. You might get away with just adding that to the perl search path ($PERL5LIB env variable will do that) because there should not have been any ABI changes between 5.8.0 and 5.8.8 but I think you would be better off installing a 5.8.8 specific mod_perl with something like
