#!/usr/local/bin/perl
use strict;
use warnings;
use XML::Twig;

my $xml = <<XML;
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
  <mml:mstyle mathsize="1em">
    <mml:mi>s</mml:mi>
    <mml:mo>+</mml:mo>
    <mml:mi>s</mml:mi>
  </mml:mstyle>
</mml:math>
XML

my $twig = XML::Twig->new (
    twig_roots => {'mml:mi' => \&subst,},    # process the element
    twig_print_outside_roots => 1,           # print the rest
);
$twig->parse ($xml);


sub subst {
    my ($twig, $value) = @_;

    print '<mml:mi>...</mml:mi>';
}

##</code><code>##

<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
  <mml:mstyle mathsize="1em">
    <mml:mi>...</mml:mi>
    <mml:mo>+</mml:mo>
    <mml:mi>...</mml:mi>
  </mml:mstyle>
</mml:math>