Something like this?

#!/usr/bin/perl -w
use warnings;
use strict;
use Tk;
use Tk::LabEntry;

my $mw = MainWindow->new;
my $vh = $mw->vrootheight;
my $vw = $mw->vrootwidth;

# this is what grabs all virtual desktops
$mw->overrideredirect(1);

# Note that the 'virtual window' height and width are $vh and $vw
# respectively, so we use those dimensions for our Canvas height 
# and width, and let the Canvas expand and fill in both x and y
# directions.
#
my $canvas = $mw->Canvas(
   -width => $vw, 
   -height => $vh, 
   -background =>'blue',
   -takefocus =>0
   ); # so canvas dosn't take focus on tab press

$canvas->pack(-expand => 1, -fill => 'both');

#just for fun instead of an image
$canvas->createRectangle(100, 100, 150, 150, -fill => 'orange'); 

my $window = 
    $canvas->Button(-text=> 'Ok',
               -command=> sub{ exit;}
    );

$canvas->createWindow($vw/2, $vh/2,
          -window=> $window );

MainLoop;
[download]

Boy, this is cool. It has gray edges (where does this gray come from? the color for window edges and title bars generated for toplevel windows by Win32 is blue on my system) but other than that it is a plain canvas as a Toplevel.