I found the problem and it has to do with the data type used. The referenced list of object instances has to be declared as a type of MS object. If I declare the variable and populate it using the following snippet it works.

$objSWbemLocator = Win32::OLE->new("WbemScripting.SWbemLocator");
$objWMIService = $objSWbemLocator->ConnectServer($strMLM,"\\root\\hewl
+ettpackard\\openview\\data");
$objNodes = $objWMIService->ExecQuery("Select * from OV_ManagedNode wh
+ere PrimaryNodeName like \"%$argNode%\"");
my $NGList = Win32::OLE::Variant->new(VT_VARIANT|VT_BYREF);
foreach $fNode (in $objNodes) 
{
    print "$fNode->{PrimaryNodeName}\n";
    $fNode->GetParents($NGArray, "TRUE") or die;
    foreach $fGrp (@{$NGArray->Value()})  
    {
        push(@nGrp, $fGrp->{Caption});
    }
}
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