#!/usr/bin/perl

use strict;
use Tk;

my $main = new MainWindow(-title => "Cooles Ding"); 

my $label = $main->Label(-text => "Cooles Ding")->pack();
my $outputframe = $main->Text(width => 80, height => 10)->pack();
my $exit_button = $main->Button(-text => "Beenden", 
                                                              -command => sub {exit} )->pack(-fill => "x");
my $start_button = $main->Button(-text => "Berechnen", 
                                                              -command => sub {my $output=cap(); $outputframe->insert($output,"insert");} )->pack(-fill => "x");


MainLoop;

sub cap { ... rechne rechne rechne ... }