Watch out for the "Great Mod_Perl Rename Debacle"... http://perl.apache.org/docs/2.0/rename.html

here is a quote:

Concurrent with these changes, a decision was also made to refuse to install mod_perl 2 in the same Perl tree as any installed mod_perl 2 package of version prior to 1.999_22.

So you need to know that mod_perl 1.999 to 1.999_21 are not usually compatible with Apache2 modules. You need to make sure that your mod_perl is >= 1.999_22 to use the Apache2 modules.