It's not a module installation issue. If it was, XML::XPath wouldn't be able to load the module, and that would result in the error Can't locate XML/XPath/NodeSet.pm in @INC.

NetWallah's right in saying the OP is incorrectly treating an XML::XPath::NodeSet object as an XML::XPath::Node::Element object. The OP should review the return value of findnodes.