Thanks so much for the troubleshooting work, and for pressing on with this as a Perl version issue, not a CentOS issue.
I already had zimbra 5.0.0GA NE on this CentOS 4.6 box, installed with the workaround of forcing an install of Scalar::Util. I didn't really want to put it into production that way, though.
I followed John's steps on my test system and it worked very smoothly. I did have stuff left in /usr/lib/perl5/5.8.8/, but it was just the List-Util package from the previous force install, so I removed it.
After a reboot, zimbra loaded up and I was able to log in.
The only immediate issue I noticed was that one zmlogger process won't stop with "service zimbra stop" (both before and after the perl downgrade). This is
already in bugzilla (18653).
Next up, reprovision the server and retest with 5.0.1.
Thanks again!
Paul