#!/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;