I found a problem while using "Tk" with "threads" together, when it is going to collect a thread (no matter "join" or "detach", no matter the thread is created by "create()" or "async{}"). 
/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;

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In reply to Tk with threads by llancet

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