The foo:thing syntax relates to namespaces. An XML parser must accept such syntax (here the node is a 'thing' in the 'foo' namespace).

The document should be considered invalid because the 'person' namespace has not been declared at the top of the XML document.

However, XML::Simple is (as it's name implies) a simplistic approach to parsing XML... it's not exactly a strict parser. It may or may not give you a warning about a missing 'prefix'.

Use a more compliant parser such as XML::Parser (directly), XML::Twig or XML::LibXML.

-David