#!/usr/bin/perl 

use strict;
use warnings;
use 5.012;
use Tk;
use Tk::DialogBox;

my $mw = MainWindow->new(-screen => $ARGV[0] || $ENV{'DISPLAY'});
$mw->geometry('200x200+-300+300');  
$mw->DialogBox(-buttons => [ 'OK' ])
->Show(-popover => $mw);

MainLoop;