Names in XML must begin with a letter. The remaining characters may be letters, digits, hyphens (-), underscores (_) or full stops (.). It is also allowable to include one colon (:) in an element name but this should only be used when working with namespaces (XML::Simple can only usefully work with namespaces when teamed with a SAX Parser).

From http://search.cpan.org/dist/XML-Simple/lib/XML/Simple.pm#SEE_ALSO:

To generate documents with namespaces, XML::NamespaceSupport is required.

Jim