Misery loves company. Found this snippet in LJ::Feed. (edit, fixed link)

    # Strip namespace from child tags. Set default namespace, let
    # child tags inherit from it.  So ghetto that we even have to do t
+his
    # and LibXML can't on its own.
    my $normalize_ns = sub {
        my $str = shift;
        $str =~ s/(<\w+)\s+xmlns="\Q$ns\E"/$1/og;
        $str =~ s/<feed\b/<feed xmlns="$ns"/;
        $str =~ s/<entry>/<entry xmlns="$ns">/ if $opts->{'single_entr
+y'};
        return $str;
    };
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I've resorted to what derby suggests for namespaced XHTML too and it works fine (this snippet is old and un-re-tested).

my $root = $doc->documentElement;
my $xpc = XML::LibXML::XPathContext->new($html);
$xpc->registerNs('x', 'http://www.w3.org/1999/xhtml');
my $htmls = $xpc->find('/x:html', $doc);
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