Rereading your post, I realize the original error message you got was "Can't locate Digest/Perl/MD5.pm in @INC".  In other words, you were trying to install the wrong module (i.e. the XS version Digest::MD5) in order to fix that problem...

Anyhow, moritz is right, Digest::MD5 already was a core module with 5.8 installations, and even with the AIX system Perl (just checked on an AIX 6.1 box that shipped with perl-5.8.2).

So, unless you're planning to do further installations of other XS modules in the future, there might be no need to go to the trouble of installing your own Perl, or some such.  All you'd need to do is find out what exactly is trying to load Digest::Perl::MD5, and edit that (plus maybe other occurrences of "Digest::Perl::MD5") to use Digest::MD5 instead.