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;
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