This works:

#!/usr/bin/env perl

use strict;
use warnings;
use XML::Simple;

my $xs = XMLin(\*DATA, ForceArray => 1, KeyAttr => { item => 'id', },)
+;
XMLout($xs, OutputFile => 'out.xml');

__DATA__
<someotherroot>
  <a id="FOOFOOFOOFOO">
    <s>555</s>
    <m>foofoo  44444</m>
    <pm>barbarbar222</pm>
    <r>nn23v</r>
    <mfg someid='2345'>xyzzy</mfg>
  </a>
</someotherroot>
[download]

You were missing the KeyAttr option in XMLin.