Make sure you are calling your 5.8 install of Perl, if you have more than one version on your system. UTF-8 in pre-5.8 versions of Perl have known problems as told by Larry Wall concering his "favorite bug to fix in 5.8". This same style of error is mentioned in the SpamAssassin FAQ here.

Whether or not that is causing your problem here, I don't know. But that might at least give you some ideas on how to further track down your problem.