sub Popup
{
 my $w = shift;
 $w->configure(@_) if @_;
 $w->idletasks;
 my ($mw,$mh) = ($w->reqwidth,$w->reqheight);
 my ($minwidth,$minheight) = $w->minsize; #JD#
 $mw = $minwidth if (defined $minwidth and $minwidth > $mw); #JD#
 $mh = $minheight if (defined $minheight and $minheight >$mh); #JD#
 my ($rx,$ry,$rw,$rh) = (0,0,0,0);
...

##</code><code>##

$mw -> minsize (qw(600 400));

##</code><code>##

use strict;
use warnings;
use Tk;

my $mw = MainWindow -> new;
$mw -> minsize (qw(600 400));
$mw -> withdraw;

for (1..10) {
	my $button = $mw -> Label (-text => "Label $_") -> pack;
}

$mw -> update;
$mw -> idletasks;
$mw -> Popup(
			-overanchor => 'c',
			-popanchor => 'c',
			);
MainLoop;