use strict;
use warnings;
use Tk;

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

my $button = $mw -> Button (
					-text => 'Center of screen',
					-command => sub {popup('screen')},
					) -> pack;

my $other_button = $mw -> Button (
					-text => 'Center of main window',
					-command => sub {popup('window')},
					) -> pack;

MainLoop;

sub popup {
	my $type = $_[0];

	my $popup = $mw -> Toplevel;
	$popup -> withdraw;

	for (1..10) {
		my $label = $popup -> Label (
					-text => "This is a label $_",
					) -> pack;
	}

	if ($type eq "screen") {
		$popup -> Popup();
	} else {

		$popup -> Popup (
				-popover => $mw,
				-overanchor => 'c',
				-popanchor => 'c',
				);
	}
}

##</code><code>##

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