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<?xml version="1.0" encoding="UTF-8"?>
<TRANSACTION>
    <FIELDS>
...
        <FIELD KEY="operation_type">do_what</FIELD>
     </FIELDS>
</TRANSACTION>

<opt>
  <password>pass&amp;word</password>
</opt>

$xmltext =~ s{(<FIELD .*?>)(.*)(</FIELD)}{$1<![CDATA[$2]]>$3}

#!perl

use HTML::Parser;
...
        <FIELD KEY="operation_type">do_what</FIELD>
     </FIELDS>
</TRANSACTION>

<TRANSACTION>
    <FIELDS>
        <FIELD KEY="user">name</FIELD>
...
        <FIELD KEY="operation_type">do_what</FIELD>
     </FIELDS>
</TRANSACTION>