in reply to Want to fetch inner most child element first

XML::XPath is old and unmaintained. I prefer XML::LibXML, which has a similar API. In XML::XSH2, a wrapper around it, you can easily do 
for &{ sort :d :k count(ancestor::*) //mml:msub } ls . ;

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Output: 

<mml:msub>swift</mml:msub>

Found 1 node(s).
<mml:msub>
           <mml:mrow>
              <mml:mi>poland</mml:mi>
           </mml:mrow>
           <mml:mrow>
       <mml:msub>swift</mml:msub>
              <mml:mi>a</mml:mi>
           </mml:mrow>
        </mml:msub>

Found 1 node(s).
<mml:msub>
     <mml:mrow>
        <mml:mi>China</mml:mi>
     </mml:mrow>
     <mml:mrow>
        <mml:msub>
           <mml:mrow>
              <mml:mi>poland</mml:mi>
           </mml:mrow>
           <mml:mrow>
       <mml:msub>swift</mml:msub>
              <mml:mi>a</mml:mi>
           </mml:mrow>
        </mml:msub>
     </mml:mrow>
  </mml:msub>

Found 1 node(s).
<mml:msub>
     <mml:mrow>
        <mml:mi>Canada</mml:mi>
     </mml:mrow>
     <mml:mrow>
        <mml:msup>
           <mml:mrow>
              <mml:mi>police</mml:mi>
           </mml:mrow>
           <mml:mrow>
              <mml:mi>bangalore</mml:mi>
           </mml:mrow>
        </mml:msup>
     </mml:mrow>
  </mml:msub>

Found 1 node(s).

[download]

Directly in XML::LibXML: 

#!/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');

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لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

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