Note that reading and writing modifed XML document using XML::Simple really works only in simple cases. The problem with XML::Simple is that XMLout(XMLin($xml)) is not guarantied to produce XML document with same structure. From perldoc XML::Simple:

       o   The API offers little control over the output of
           "XMLout()".  In particular, it is not especially
           likely that feeding the output from "XMLin()" into
           "XMLout()" will reproduce the original XML (although
           passing the output from "XMLout()" into "XMLin()"
           should reproduce the original data structure).
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--
Ilya Martynov (http://martynov.org/)