#!/usr/bin/perl
use strict;
use warnings 'all';
use Tk;
use threads;

my $thr = threads->new(sub { (print("An!\n"), sleep 1) while 1 });
my $mw  = tkinit;

$mw->Button(-text => "Start", -command => sub { threads->yield; $thr->join })->pack;
$mw->Button(-text => "Stop!", -command => sub { $thr->detach })->pack;

MainLoop;