Clearing the PERL5LIB environment variable should take care of that.
As it stands, I think your 'ppm' is loading perl-5.12.4 - and perl-5.12.4 is finding the 5.8.8 Config.pm (because of the PERL5LIB setting).
That 5.8.8 Config.pm correctly reports that the perl executable is version 5.8.8 ... and you get that mismatch error that you reported.
Remove that PERL5LIB setting and all should be fine.