#!/usr/bin/perl
use warnings;
use strict;
use threads;
use threads::shared;

# declare, share then assign
my $ret;
share $ret;
$ret = 0;
my $val = 0;

#create thread before any tk code is called
my $thr = threads->create( \&worker );

use Tk;
my $mw = MainWindow->new();

    my $label = $mw->Label(
         -width => 50,
         -textvariable => \$val )->pack();
 
    my $timer = $mw->repeat(10,sub{
              $val = $ret;
            });

MainLoop;

# no Tk code in thread
sub worker {
   for(1..10){
     print "$_\n"; 
     $ret = $_;
     sleep 1; 
    }
   $ret = 'thread done, ready to join';
   print "$ret\n";
}