No, any way you slice it, you're taking a screenshot, but AFAIK, Tk exposes no API to retrieve said information.

http://search.cpan.org/grep?cpanid=SREZIC&release=Tk-804.030&string=pixel&i=1&n=1&C=0

update: well, it might be possible with Tk::WinPhoto on linux, but it doesn't expose

GetDesktopWindow http://search.cpan.org/grep?cpanid=SREZIC&release=Tk-804.030&string=desktop&i=1&n=1&C=0

So at the very least you'd need X11::GuiTest

Probably easiest to use Imager::Screenshot

See also Gtk2-screenshot, Tk Screen and Canvas Screenshots