#Private Declare Function ReleaseDC Lib "user32" (ByVal hWnd As Long, ByVal hDC As Long) As Long
Win32::API->Import(
    'user32', 'long ReleaseDC(long hWnd, long hDC)',
);

#Private Declare Function GetDesktopWindow Lib "user32" () As Long
Win32::API->Import(
    'user32', 'long GetDesktopWindow()',
);

sub PixelColor {
  my ($hwnd, $x, $y) = @_;
  my $dc = GetDC($hwnd);
  my $rgb = GetPixel($dc, $x, $y);
  ReleaseDC($hwnd, $dc);
  return $rgb < 0 ? undef: sprintf "%06x", $rgb;
}

print PixelColor(GetDesktopWindow(), 100, 100);