use XML::Writer;
use IO::File;
$fh = IO::File->new();
open($fh, '>', \$doc);
$xml = XML::Writer->new(DATA_MODE => 1, DATA_INDENT => 2, OUTPUT => $fh);
$xml->startTag('root');
$xml->emptyTag('elem', attr => '®');
$xml->endTag('root');
$xml->end;
print $doc;

##</code><code>##

<root>
  <elem attr="&amp;reg;" />
</root>

##</code><code>##

<root>
  <elem attr="&reg;" />
</root>