There is an ongoing outage on the primary CPAN mirror. It is possible to work around the issue by using MetaCPAN as a mirror.

Provides

in lib/GraphViz/Diagram/ClassDiagram/Attribute.pm
in lib/GraphViz/Diagram/ClassDiagram/ClassElement.pm
in lib/GraphViz/Diagram/ClassDiagram/GlobalVar.pm
in lib/GraphViz/Diagram/ClassDiagram/Method.pm