Based on my experience, there are 4 gotchas that need to be looked at. First, go through the httpd.conf and replace PerlModule Apache::SomeCoreModule with PerlModule Apache2::SomeCoreModule. This needs to be done for every directive. Second, the same thing has to be done with the mod_perl : Change  mod_perl to  mod_perl2. Third, has CGI been upgraded? Fourth, the one that I always seem to forget---you'll have to set the environmental variable for mod_perl:

$ENV{MOD_PERL_API_VERSION} to the version that you are using