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According to this, optional arguments are 'filled in' to their default values, by the compilers of languages that support them. It also says that the default values should be clearly documented.
The upshot appears to be that you will have to pass the default values when calling from a language that does not support this behaviour. So, you will need to look up the dfault values and provide them explicitly either every time you call via Win32::API or supply a wrapper function that fills them in.
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In reply to Re^3: functions with optional arguments in Windows DLLs