#!/usr/bin/perl
use warnings;
use strict;

use XML::LibXML;
my $xml = 'XML::LibXML'->load_xml( location => '1.xml' );
for my $element ($xml->findnodes('/ANNOTATION_DOCUMENT/TIER'
                                 . '[@LINGUISTIC_TYPE_REF="orthT"]/ANNOTATION/'
                                 . 'REF_ANNOTATION/ANNOTATION_VALUE/text()')
                ) {

    # Replace by your logic, or better, see what AnomalousMonk wrote.
    (my $translit = $element) =~ s/m/M/g;

    $element->setData($translit);
}
print $xml;