$NRE_ID = $XMLref->{'diffgr:diffgram'}->{'NewDataSet'}->{'Table'}->[0]->{'CLIENT_ID'};

or 

$NRE_ID = $XMLref->{'diffgr:diffgram'}->{'NewDataSet'}->{'Table'}->{'CLIENT_ID'};

##</code><code>##

$XMLref = XMLin($returnStr, suppressempty => '');
$xmlstream = Dumper($XMLref);