It looks like you want to remove namespaces, and don't want to mess with validating the XML with a DTD

# suppressing DTD validation and errors due to no DTD validation
my $parser = XML::LibXML->new(load_ext_dtd => 0, expand_entities => 0,
+ suppress_errors => 1);
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From XML::LibXML::Element:

setAttributeNS

  $node->setAttributeNS( $nsURI, $aname, $avalue );

Namespace-aware version of setAttribute, where $nsURI is a namespace URI, $aname is a qualified name, and $avalue is the value. The namespace URI may be null (empty or undefined) in order to create an attribute which has no namespace.

or...

removeAttribute

  $node->removeAttribute( $aname );

The method removes the attribute $aname from the node's attribute list, if the attribute can be found.