The spec file for perl-libwww-perl should have the line BuildRequires: perl(URI) since the URI module is required to build and test libwww-perl.
Please verify, is this still an issue in FC4?
The URI module is a build requirement. Handled by the new patch submitted in #150363. (https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=150363)