Check out the noattr option to XMLout:

% perl -w
use XML::Simple;

$xml = XMLin("pre.xml");
$xml->{core}->{test} = 'testeroo';
print XMLout($xml, noattr => 1);
^D
<opt>
  <core>
    <test>testeroo</test>
    <variable>ere</variable>
  </core>
</opt>
<
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Update: You may also be interested in the rootname => 'config' option.