XML::LibXML::XPathContext exists since XML::LibXML 1.61, and it's the appropriate way of working with namespaces in XPaths for XML::LibXML.

my $xpc = XML::LibXML::XPathContext->new( $doc );
$xpc->registerNs( ota => 'http://www.opentravel.org/OTA/2003/05' );
my @options = $xpc->findnodes('//ota:OriginDestinationOption');
[download]

How did one do the above before XML::LibXML 1.61?

# Doesn't work because OriginDestinationOption belong to
# the http://www.opentravel.org/OTA/2003/05 NS, not the
# null NS.

my $root = $doc->documentElement();
my @opts = $root->findnodes("//OriginDestinationOption");
[download]

If there was a prefix in play, I believe I could specify it, but the elements are using the default namespace.

# Doesn't work because OriginDestinationOption element
# appears as <OriginDestinationOption>, rather than
# <ota:OriginDestinationOption>

my $root = $doc->documentElement();
my @opts = $root->findnodes("//ota:OriginDestinationOption");
[download]

The only thing I've found that works is

my $root = $doc->documentElement();
my @opts = $root->findnodes("//*[name()="OriginDestinationOption"]');
[download]

yuck!