Win32::API::Prototype is not part of Win32::API. Win32::API::Struct is part of Win32::API. I should have a new CPAN release of Win32::API with Perl 5.6 support by the end of the week. pack's syntax in 5.6 is very different/older compared to 5.8/Modern Perl so many tests fail either because (), [], !, and J are not supported by 5.6 pack().