You forgot the call to ReleaseDC(). You might just have created a huge memory leak. And what on earth are you trying to pass on as a hWnd? That's the ID (window handle) of the control/window you're trying to access, so try to get a valid value. If you want to access the desktop as a window, you can use the GetDesktopWindow() API call, which will return the hWnd of the mother of all windows, the desktop. You can proceed from there. These are the additional/changed code snippets: 
#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);

[download]
Well, that's the theory. In practice I always get "ffffff" undef back, which means this pixel can't be read — likely because the pixel data isn't buffered. Oh well. Perhaps it works better with a hWnd of an actual window...

p.s. This hWnd should be a property of your control or canvas, or whatever terminology you might be used to.

p.p.s. For people who feel like trying this out, make sure you have at least version 0.40 of Win32::API, or you'll get a deep recursion error on the Import method — something to do with AUTOLOAD, because that method doesn't exist. Needless to say, it won't work, either.

In reply to Re: Re: Detect Pixel Color on WinXP by bart
in thread Detect Pixel Color on WinXP by kesterkester

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