#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use XML::LibXML;

my $xml = 'XML::LibXML'->load_xml(location => '/path/to/file.xml');

say for map $_->[0],
        sort { $b->[1] <=> $a->[1] }
        map [ $_, $_->findvalue('count(ancestor::*)')],
        $xml->findnodes('//mml:msub');