Tk is not thread safe, that is why you get the GUI freeze you described in the node above. You cannot directly access a Tk widget from a thread. 

Workarounds are available. The most foolproof way is to have a shared variable in the thread which gets set to a value signalling the main Tk thread that it's time to change the button text, which ikegami showed. Tk must run a timer to watch that shared variable, here is a simpler version to demonstrate.

#!/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";
}
[download]