use warnings;
use strict;
use XML::LibXML::Reader;

my $xml = <<'ENDXML';
<foo xmlns="http://www.example.com"><bar/></foo>
ENDXML

my $reader = XML::LibXML::Reader->new(string => $xml);
while ($reader->read) {
	my $node = $reader->copyCurrentNode;
	$node->setNamespaceDeclURI(undef, undef)
		if $node->nodeName eq 'bar';
	print "$node";
}

__END__

<foo xmlns="http://www.example.com"/><bar/><foo xmlns="http://www.example.com"/>