.../auto/DBI/DBI.so: undefined symbol: PL_tainting

Presumably, the Perl interpreter that mod_perl has been linked against is not the one that DBI.so has been built/linked against. In other words, DBI.so requires/references a symbol (PL_tainting) that isn't provided by that Perl interpreter.

It's a little difficult to diagnose such issues remotely, because "the Perl interpreter" can come in different flavors (dynamically or statically linked etc.)... but typically, mod_perl.so is linked dynamically against a libperl.so (which in this case is the Perl interpreter) — and due to (presumably) being a different version, it might not be providing the symbol PL_tainting.

You can use ldd to check which libs mod_perl.so depends on. You should see something like:

$ ldd /usr/lib64/apache2/mod_perl.so 
        linux-vdso.so.1 =>  (0x00007fffb37ff000)
        libperl.so => /usr/lib/perl5/5.10.0/x86_64-linux-thread-multi/
+CORE/libperl.so (0x00007fc9cd343000)
        libm.so.6 => /lib64/libm.so.6 (0x00007fc9cd0ba000)
        libdl.so.2 => /lib64/libdl.so.2 (0x00007fc9cceb6000)
        libcrypt.so.1 => /lib64/libcrypt.so.1 (0x00007fc9ccc79000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fc9cca5c000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fc9cc703000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fc9cd93f000)
[download]

You can then run nm on the libperl.so being pulled in — e.g. (in my case):

$ nm -D /usr/lib/perl5/5.10.0/x86_64-linux-thread-multi/CORE/libperl.s
+o | grep PL_tainting
[download]

nm lists what symbols a shared lib (or binary) uses. The symbols required but not provided by the lib itself are marked with "U" (=undefined), while the symbols provided are marked with D/B/T (depending on which segment they belong to). For example, when you run nm on your DBI.so, you should see

$ nm -D /usr/lib64/perl5/vendor_perl/5.8.8/x86_64-linux-thread-multi/a
+uto/DBI/DBI.so | grep PL_tainting
                 U PL_tainting
[download]

And my guess would be that whatever your mod_perl is pulling in is not providing that symbol.  In that case, you would need to find another mod_perl.so that matches your Perl installation. Or install a Perl that matches your existing mod_perl.so (whatever is less trouble, because mod_perl.so must also match your existing Apache version...).

HTH.