/usr/bin/perl
use Tk;
use threads;
use strict;

my $mw=MainWindow->new(-title=>'test');
my $button=$mw->Button(
    -text    => 'show the problem',
    -command => sub{
        print "button: ouch!\n";
        my $thread=async{print "inside thread\n";};
        print "before join\n";
        $thread->join;
    }
);

MainLoop;