$txt1
<?xml version="1.0" encoding="utf-8" ?>
     <D:propfind xmlns:D="DAV:">
       <D:propname/>
     </D:propfind>

$txt1 $xpath1 
Failed $xpath /DAV:propfind/DAV:propname
$txt1 $xpath2 
XML::LibXML::Document::findnodes('/propfind/propname') (NOT XPathContext) 0 nodes
XML::LibXML::XPathContext::findnodes('/propfind/propname') 0 nodes
$txt1 $xpath3 
XML::LibXML::Document::findnodes('/D:propfind/D:propname') (NOT XPathContext) 1 nodes
XML::LibXML::XPathContext::findnodes('/D:propfind/D:propname') 1 nodes
$txt2
<?xml version="1.0" encoding="utf-8" ?>
     <propfind xmlns="DAV:">
       <propname/>
     </propfind>

$txt2 $xpath1 
Failed $xpath /DAV:propfind/DAV:propname
$txt2 $xpath2 
XML::LibXML::Document::findnodes('/propfind/propname') (NOT XPathContext) 0 nodes
XML::LibXML::XPathContext::findnodes('/propfind/propname') 0 nodes
$txt2 $xpath3 
Failed $xpath /D:propfind/D:propname