Since you are using this as a screensaver type app, and you want it to have focus when it's out of the mainwindow, you may want a grab_global call. Try this. I've included an 'Escape' key to quit, but you could use a password. You could also make it full screen and put a graphic on it.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

my $mw = MainWindow->new;

#my $width =  $mw->screenwidth; 
#my $height = $mw->screenheight; 
#$mw->geometry($width . 'x' . $height .'+0+0'); 

$mw->bind('<Motion>', [\&activity ,'mouse']);
$mw->bind('<Key>', [ \&activity, Ev('K')]);

$mw->after(500,sub {$mw->grabGlobal}); #window must be visible 
$mw->focusForce;                       #for grab 

MainLoop;

sub activity {
    if( $_[1] eq 'Escape'){ exit }
  print chr(07); #beep 
}
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