Taking a stab in the dark: it sounds as if your mod_perl and/or Apache and/or perl were not compiled in a compatible way. One wiser than I could probably give you more detail, but I'd assume that if you "rolled your own" perl, that perhaps one of the following is happening:
- The versions of perl / libperl / mod_perl mismatch
- libperl is not on the LD_LIBRARY_PATH (or see /etc/ld.so.conf) of Apache (thus mod_perl)
One test might be to invoke perl directly and see if DBI.pm can be used from the shell, ie. run some DBI-based script interactively and look for similar errors.