First all, thanks, those are great suggestions, I didn't know about the setNamespace trick for defining context, however I still have to redefine it for every level as I progress through the xml right? Perhaps I'm not being clear. Incomming example:

<root xmls='urn:foo'>
    <first_sub name='foo'>
        <second_sub id='1'>
            <third_sub>Foo</third_sub>
        </second_sub>
        <second_sub id='2'>
        </second_sub>
    </first_sub>
</root>
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Now just imagine that there are multiple first_sub, second_sub, and third_sub elements nested in the example above. In order to get at all the values I want, as far as I can tell I have to do something like this(assuming $xml is set to the above):

my $parser = XML::LibXML->new();
my $data = $parser->parse_string ( $xml );
$data->setNamespace ( 'urn:foo', 'x' );

for my $first_sub ( $data->findnodes ('/x:root/x:first_sub')) {
    my $name = $first_sub->getAttribute('name');

    $first_sub->setNamespace ('urn:foo','x');
    for my $second_sub ( $first_sub->findnodes ('./x:second_sub')) {
        my $id = $second_sub->getAttribute('id');

        $second_sub->setNamespace ('urn:foo','x');
        for my $third_sub ( $second_sub->findnodes('./x:third_sub')) {
             # do something with the values
        }
    }
}
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So I'm pretty sure its confirmed that I can't remove the name space to avoid all this setting/defining and extra work in the XPath expressions, which was my original question, this is fine, I'll just stick to something like the above. thanks again.