Why do I have problems with mod_perl and Apache?
Because you didn’t install it according to the instructions, perhaps using older or beta versions of the software or prebuilt binaries or packages, or you don’t have all the Perl modules installed properly, or because you are using PHP (as many people have reported problems using PHP and mod_perl both with Apache, and there’s not much we, as lowly application programmers, can do about that :-).