my $xp = XML::XPath->new(filename => $file);
$plantdata->{nodeset} = $xp->find('//loop_device[@loop_number="1"]');

##</code><code>##

foreach my $device ($plantdata->{nodeset}->get_nodelist()) {
  $devicename = find('devicename')->string_value;
  $devicetype = find('devicetype')->string_value;

  do more stuff
}

##</code><code>##

if ($devicetype eq "complex") {
  $seachstring = sprintf "//loop_device[@devicename=\"%s\"]/subdevice", $devicename;
  $subdevice = $xp->find($searchstring);
}

##</code><code>##

$subdevice = $device->find('/subdevice');