I was just trying this as well with the same problem. Since most of the values I am trying to write are VT_R4, I changed the line 1559 in the OPC.pm module from: my $value = Variant(VT_VARIANT|VT_BYREF, shift); to my $value = Variant(VT_R4|VT_BYREF, shift); and voila!! it works fine. And this mod begs the question, What about other types?...