for &{ sort :d :k count(ancestor::*) //mml:msub } ls . ;
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Output:

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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Welcom dharan,

"...using XML::XPath..."

And the code that you are having a problem with is where?

Regards...Ed

Thanks flex for swift reply, below is the code and not sure how to proceed further.

 use XML::XPath;
 my $xp = XML::XPath->new(filename => 'mathml.txt');
 my $nodeset = $xp->find('/mml:math/*');

 foreach my $node ($nodeset->get_nodelist) {
    $l = XML::XPath::XMLParser::as_string($node),
}
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The XPath expression that you have written merely matches the outermost node.   What you might want to use here are axes.

Try something like this:   (extemporaneous answer, check it yourself)

//xml:msub[count(descendant::xml:msub) = 0] to retrieve all msubs who do not have any descendants of the same type.   These must be the leaves of the structure.

Now, for each of these, perform a separate query for ancestor::xml:msub which will return an ordered list (from closest to farthest, or is it the other way around?) of all ancestors back to the top of the tree.

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1142802
# want inner child first

use strict;
use warnings;

$_ = join '', <DATA>;
my @elements;

s/<mml:msub>\K(\w+)(?=<\/mml:msub>)/<A>$1<\/A>/g;

push @elements, $1 while
  s/(<mml:msub>(?:(?!<mml:msub>).)*?<\/mml:msub>)/ '<' . @elements . '
+>'/se;

for my $n (1..@elements)
  {
  local $_ = $elements[$n - 1];
  1 while s/<(\d+)>/$elements[$1]/;
  print "$n:  $_\n\n";
  }

__DATA__
<mml:math> 
  <mml:mi>Goa</mml:mi>
  <mml:mo>abu</mml:mo>
  <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>

  <mml:mo>miot</mml:mo>
  <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>
</mml:math>
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hehehe