I'm just following the MS documentation on Get DC (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/gdi/devcons_4esj.asp):

HDC GetDC(<br />
  HWND hWnd   // handle to window<br />
);
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< Parameters
hWnd
[in] Handle to the window whose DC is to be retrieved. If this value is NULL, GetDC retrieves the DC for the entire screen.

---

I missed the part about ReleaseDC so thanks for the heads up. I suspect that you are getting undef because of your call to GetDesktopWindow.

Here is the code I used to test with the ReleaseDC and check for negative numbers added in.:

#!/usr/bin/perl
use strict;
use warnings;
use Win32::API;

Win32::API::Struct->typedef( POINT => qw(
  LONG x;
  LONG y;
) );

Win32::API->Import( 'user32' => 'BOOL GetCursorPos(LPPOINT pt)' );
Win32::API->Import( 'user32' => 'long GetDC(long hWnd)' );
Win32::API->Import( 'gdi32'  => 'long GetPixel(long hWnd, long x, long
+ y)' );
Win32::API->Import( 'user32' => 'long ReleaseDC(long hWnd, long hDC)' 
+);

sub PixelColor {
  # if $hwnd is null or '', act on the entire screen
  my ($hwnd, $x, $y) = @_;
  my $dc = GetDC($hwnd);
  my $rgb = GetPixel($dc, $x, $y);
  ReleaseDC($hwnd, $dc);
  return $rgb < 0 ? undef: sprintf "%06x", $rgb;
}

my $pt = Win32::API::Struct->new( 'POINT' );

while (1) {
  GetCursorPos($pt) or die "GetCursorPos failed: $^E";
  print "Cursor is at: $pt->{x}, $pt->{y}\n";
  print "The pixel color is: ", PixelColor('',$pt->{x}, $pt->{y}), "\n
+";
  sleep(2);
}
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Updated: Added code and suspicion of cause of bart's problem with the code, so go back and reread this entire post. :-)