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use XML::Simple;

...
$res = XMLout($xml);

print $res;

<config>
<core>
    <variable>ere</variable>
</core>
</config>

<opt>
  <core test="testeroo" variable="ere" />
</opt>

<opt>
  <core>
...
    <variable>ere</variable>
  </core>
</opt>