#!/usr/bin/perl -w
use strict;
use Tk;

my $w = MainWindow->new;

my $button;

$button = $w->Button(-text => "click me", -command => sub
{
    my $mw = $w->Toplevel;
    # here I need somehow position the $mw over the $button
    $mw->Label(-text => "hello")->pack();
})->pack;

MainLoop; 

##</code><code>##

    my @wg = ($w->geometry =~ /\+(\d+)\+(\d+)/);
    my @bg = ($button->geometry =~ /\+(\d+)\+(\d+)/);
    $wg[$_] += $bg[$_] for (0..@wg-1);
    $mw->geometry("+$wg[0]+$wg[1]");