You can write xpaths like this

my $firstWeightXpath = qq{
  //*:data[
    attribute:key="$weightKey"
  ]
  [0]
};
[download]

I use this helper , allows you to use it on dom and nodes, like

$dom->F(
  '//what:ever', 
   'what' => 'urn:yeahwhatever',
   'that' => 'urn:thatalso',
)->[0]->F('//what:else/that:there');

sub XML::LibXML::Node::F {
    my $self   = shift;
    my $xpath  = shift;
    my %prefix = @_;
    our $XPATHCONTEXT;
    $XPATHCONTEXT ||= XML::LibXML::XPathContext->new();
    while( my( $p, $u ) = each %prefix ){
        $XPATHCONTEXT->registerNs( $p, $u );
    }
    $XPATHCONTEXT->findnodes( $xpath, $self );
}
[download]

Example of usage at Re^2: XML namespace question

I also use xpather.pl