Adding node to a container (group) with Perl?

I'm attempting to add a specific nodeID to a container using the Perl SDK.

None of the following seem to be working... I must be missing something obvious!

my %UPDATEGROUP = (

   'ContainerID' => $_[1],

  'MemberPrimaryID' => $_[2]

  );

my $swis = SW::InformationService->new();

$swis->outputxml("true");

$swis->proxy("https://".$hostname.$endpoint);

$swis->proxy->http_request->authorization_basic($username, $password);

return $swis->Create('Orion.Container', \%UPDATEGROUP);

..or

return $swis->Create('Orion.ContainerMembers', \%UPDATEGROUP)

Any help / guidance woudl be very much appreciated.

Thanks,

Graeme