#!/usr/bin/perl -w
use strict;
my $incoming_xml = '
<?xml version="1.0" encoding="UTF-8"?>
<TRANSACTION>
    <FIELDS>
        <FIELD KEY="user">name</FIELD>
        <FIELD KEY="password">pass&word</FIELD>
        <FIELD KEY="operation_type"><do_what></FIELD>
     </FIELDS>
</TRANSACTION>';
print $incoming_xml;
print "\n==============\n";
$incoming_xml =~ s/(<FIELD[^>]*>)(.*)(<\/FIELD>)/$1.&html_transliterate($2).$3/eg;
print $incoming_xml;
exit;
sub html_transliterate{
	my $in_str = shift;
	$in_str =~ s/&/!38/g;
	$in_str =~ s/</!40/g;
	$in_str =~ s/>/!41/g;
	return $in_str;
}

##</code><code>##

<?xml version="1.0" encoding="UTF-8"?>
<TRANSACTION>
    <FIELDS>
        <FIELD KEY="user">name</FIELD>
        <FIELD KEY="password">pass&word</FIELD>
        <FIELD KEY="operation_type"><do_what></FIELD>
     </FIELDS>
</TRANSACTION>
==============

<?xml version="1.0" encoding="UTF-8"?>
<TRANSACTION>
    <FIELDS>
        <FIELD KEY="user">name</FIELD>
        <FIELD KEY="password">pass!38word</FIELD>
        <FIELD KEY="operation_type">!40do_what!41</FIELD>
     </FIELDS>
</TRANSACTION>