What is this all about?

Looks like a bug - this one looks like it might be related to http://rt.cpan.org/Public/Bug/Display.html?id=6757 which refers you to http://www.perlmonks.org/index.pl?node_id=369601. As mentioned in the update to that perlmonks post, there seems to be some problem with the member alignment aspect of the computation - which becomes apparent when you turn on $Win32::API::DEBUG (as recommended in an earlier post). You can handle structs in Win32::API without having to resort to using Win32::API::Struct. I think that not using Win32::API::Struct is probably the best way to workaround any problems that Win32::API::Struct throws at you :-)

Cheers,
Rob