I use this in one of my programs to read parts of the screen. It's not Perl but Pascal, but you should be able to adapt it, I hope.

procedure TForm1.ScreenShot(wnd: HWND; x : integer; y : integer; Width
+ : integer; Height : integer; bm : TBitMap);
var
  dc: HDC; lpPal : PLOGPALETTE;
begin
{test width and height}
  if ((Width = 0) OR (Height = 0)) then exit;
  bm.Width := Width;
  bm.Height := Height;
{get the screen dc}
  dc := GetDc(wnd);
  if (dc = 0) then exit;
{do we have a palette device?}
  if (GetDeviceCaps(dc, RASTERCAPS) AND RC_PALETTE = RC_PALETTE) then
  begin
    {allocate memory for a logical palette}
    GetMem(lpPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY)))
+;
    {zero it out to be neat}
    FillChar(lpPal^, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY
+)), #0);
    {fill in the palette version}
    lpPal^.palVersion := $300;
    {grab the system palette entries}
    lpPal^.palNumEntries :=GetSystemPaletteEntries(dc,0,256,lpPal^.pal
+PalEntry);
    if (lpPal^.PalNumEntries <> 0) then
    begin
      {create the palette}
      bm.Palette := CreatePalette(lpPal^);
    end;
    FreeMem(lpPal, sizeof(TLOGPALETTE) + (255 * sizeof(TPALETTEENTRY))
+);
  end;
  {copy from the screen to the bitmap}
  BitBlt(bm.Canvas.Handle,0,0,Width,Height,Dc,x,y,SRCCOPY);
  {release the screen dc}
  ReleaseDc(wnd, dc);
end; (* ScreenShot *)
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BTW: I think you can ignore the (lengthy) part about palettes, unless you want/need your program to work with 256-color display modes. That'd shorten it to:

procedure TForm1.ScreenShot2(wnd: HWND; x : integer; y : integer; Widt
+h : integer; Height : integer; bm : TBitMap);
var
  dc: HDC;
begin
  {test width and height}
  if ((Width = 0) OR (Height = 0)) then exit;
  bm.Width := Width;
  bm.Height := Height;
  {get the screen dc}
  dc := GetDc(wnd);
  if (dc = 0) then exit;
  {copy from the screen to the bitmap}
  BitBlt(bm.Canvas.Handle,0,0,Width,Height,Dc,x,y,SRCCOPY);
  {release the screen dc}
  ReleaseDc(wnd, dc);
end; (* ScreenShot *)
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