I have read Johns announcement again and don't understand the following :-
"To all who are having an issue with Scalar::Util on 5.0-
We've identified that the issue is not with Zimbra, but rather CentOS."
From this thread
#27598: XS version error from Compress:Zlib it would seem that the problem has been around for a while, including on RedHat.
Hence why Scalar::Util has been included in the GA 5 release for RHEL4, and in one of the bug reports
Bug 22466 - 20071204022445 build multinode install failure will be included in all other releases.