That process you describe works fine for me - I don't know why you would be getting the error you reported. (I wonder if it was a deficiency in the typemaps at the time.) The XS file should look like this: 
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"

int fmode( FILE *stream ) {
    return stream->_flag;
}

MODULE = Win32::Fmode    PACKAGE = Win32::Fmode

PROTOTYPES: DISABLE

int
fmode (stream)
    FILE *stream

[download]
If it were a portable cross-platform solution I'd put it on cpan (though not under the Win32 namespace, of course) .... pity ...

Cheers,
Rob

In reply to Re^2: F_GETFL and Win32 by syphilis
in thread F_GETFL and Win32 by dragonchild

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