Base class package "Tree::DAG_Node" is empty

Today I ran into this error:

Base class package “Tree::DAG_Node” is empty.     (Perhaps you need to ‘use’ the module which defines that package first.)  at /usr/lib/perl5/vendor_perl/5.8.8/XML/Validator/Schema/ line 2

An odd error from perl’s XML Validation libraries; here is how you fix it: install the Tree::DAG_Node package

yum -y install perl-DAG_Node
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