My original code, the one with the problem, was

Win32::API->Import(
   'kernel32',
   'BOOL PeekNamedPipe(
      HANDLE  hNamedPipe,
      LPVOID  lpBuffer,
      DWORD   nBufferSize,
      LPDWORD lpBytesRead,
      LPDWORD lpTotalBytesAvail,
      LPDWORD lpBytesLeftThisMessage
   )',
);
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There's no way to pass NULL when using that syntax. Using that syntax is what sets has_proto. I may not have found the optimal *workaround*, but that doesn't mean the bug isn't there. I didn't use LPLPPP because I assumed it was buggy too.

More depression when I see people proffering this kind unnecessary complexity:

If you think LPLPPP is shorter...

BEGIN {
   # BOOL WINAPI PeekNamedPipe(
   #    __in       HANDLE hNamedPipe,
   #    __out_opt  LPVOID lpBuffer,
   #    __in       DWORD nBufferSize,
   #    __out_opt  LPDWORD lpBytesRead,
   #    __out_opt  LPDWORD lpTotalBytesAvail,
   #    __out_opt  LPDWORD lpBytesLeftThisMessage
   # )

   my $f = Win32::API->new('kernel32', 'PeekNamedPipe', 'LPLPPP', 'L')
      or die $^E;

   sub PeekNamedPipe {
      my $vBuffer           = defined($_[1]) ? $vBuffer          : 0;
      my $nBytesRead        = defined($_[3]) ? pack('L!', $_[3]) : 0;
      my $nTotalBytesAvail  = defined($_[4]) ? pack('L!', $_[4]) : 0;
      my $nBytesLeftThisMsg = defined($_[5]) ? pack('L!', $_[5]) : 0;

      my $rv = $f->Call(
         $_[0],
         $vBuffer,
         $_[2],
         $nBytesRead,
         $nTotalBytesAvail,
         $nBytesLeftThisMsg,
      );

      $_[1] = $vBuffer                         if defined $_[1];
      $_[3] = unpack('L!', $nBytesRead       ) if defined $_[3];
      $_[4] = unpack('L!', $nTotalBytesAvail ) if defined $_[4];
      $_[5] = unpack('L!', $nBytesLeftThisMsg) if defined $_[5];

      return $rv;      
   }
}
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That's why I prefer to use the prototype approach. It's much simpler. It handles packing pointers. It just doesn't handle NULL.

I agree with you. Avoiding this complexity would be much better, but a bug is preventing me.