I am not completely clear about your intention and may repeat something you already know:

XML::LibXML::XPathContext->registerNs doesn't modify the context node (document) in any way, it only maps the prefixes to namespaces. This mapping has nothing common with namespace mapping in any element.

When looking for nodes via:

$xc->findnodes('//*/info/fooTransaction/transactionDetail/histFile:tra
+nsactionSummary/*')
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the namespace qualified element names are compared. In xpath context the histFile:transactionSummary is mapped to <urn:histFile>transactionSummary, while the fully qualified name of histFile::transactionSummary element in your document is <http://yet.another.arbitrary.url/>transactionSummary, so they don't match.

In XPath you can also look by local or qualified name of the element.

# looking by local-name
$xc->findnodes('//*/info/fooTransaction/transactionDetail/*[local-name
+() = "transactionSummary"]/*');

# looking by qualified name
$xc->findnodes('//*/info/fooTransaction/transactionDetail/*[name() = "
+histFile:transactionSummary"]/*');
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In this case you can call findnodes method on any node, you don't need the XML::LibXML::XPathContext with its prefix => namespace mapping:

$doc->findnodes('//*/info/fooTransaction/transactionDetail/*[name() = 
+"histFile:transactionSummary"]/*');
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