In addition to what Corion just said...

5.8.0 is really the first version of Perl 5.8, so it's very likely that there are more bugs in it, than in any following perl version.

May DBI requires a feature that was not yet in perl 5.8.0, though it's also possible that there was a particular bug in perl getting on Tim Bunce's nerves, and instead of working around it, he requires that you use a version of perl where that bug is fixed.

So, instead of trying to make DBI work on perl 5.8.0, you're likely better off upgrading perl (but I recommend you stay in 5.8.x, for the smoothest upgrade experience).