It works, but from the DOM perspective, the nodes created by createElement don't seem to have any namespace attached

use strict;
use XML::LibXML;

my $ns = 'http://www.example.com';

my $doc =  XML::LibXML::Document->new;
$doc->setDocumentElement($doc->createElementNS($ns, 'q:one'));
my $elem_two  = $doc->documentElement->appendChild($doc->createElement
+NS($ns, 'q:two'));
my $elem_three = $doc->documentElement->appendChild($doc->createElemen
+t('q:three'));
warn $doc->toString(1);

# look at the namespace in DOM
warn 'namespace of q:two   is ', $elem_two->namespaceURI, "\n";
warn 'namespace of q:three is ', $elem_three->namespaceURI, "\n";

# via findnodes
warn 'q:* nodes are ', join (',', map {$_->nodeName} $doc->documentEle
+ment->findnodes('//q:*')), "\n";
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The result is:

<?xml version="1.0"?>
<q:one xmlns:q="http://www.example.com">
  <q:two xmlns:q="http://www.example.com"/>
  <q:three/>
</q:one>
namespace of q:two   is http://www.example.com
namespace of q:three is
q:* nodes are q:one,q:two
[download]

Moreover the manual of XML::LibXML::DOM states:

It is also important to repeat the specification: While working with namespaces you should use the namespace aware functions instead of the simplified versions.