# LibXML lingo; XPath expr is off the cuff, untested
$doc->findvalue('/section[last()]/title[1]/@num');

##</code><code>##

$doc->root(sub {
    $_->last_child( 'section', sub {
        $_->first_child( 'title', sub {
            $_->att( 'num')
        })
   })
});