Short answer, it is a bug with the latest version of XML::SAX (specifically the XML::SAX::PurePerl parser module in that distribution):

http://aspn.activestate.com/ASPN/Mail/Message/Perl-XML/2927108

You probably don't want to use the PurePerl parser anyway for performance reasons. The simplest fix is to install XML::SAX::Expat. That will then become the default SAX parser. Once you've done that, try the XML::Simple install again. The errors should be gone.