Hi, "fork and join" are sort of 2 contradictory terms. Join is commonly used with threads, which on Windows emulates a Perl fork. Forks don't join, they exit with a return code. You don't say if you are on Windows or a Linux based system. You also don't say what exactly your forked subroutines will be doing, how will they signal that they are done? If you are on Windows and running long subroutines in an asynchronous matter, you may want to use threads. Threads with Tk need very special care, but can be done. Here are a couple of options, unless you want to provide more specifics as to what you are doing.

Using Tk::ExecuteCommand, you can change the text of the ExecuteCommand module's default buttons to anything you want.

#!/usr/bin/perl -w
use Tk;
use Tk::ExecuteCommand;
use Tk::widgets qw/LabEntry/;
use strict;

my $mw = MainWindow->new;

my $ec = $mw->ExecuteCommand(
    -command    => '',
    -entryWidth => 50,
    -height     => 10,
    -label      => '',
    -text       => 'Execute',
                  )->pack;
$ec->configure(-command => 'date; sleep 10; date');


my $button = $mw->Button(-text =>'Do_it',
                         -background =>'hotpink',
                         -command => sub{ $ec->execute_command },
                        )->pack;
 
MainLoop;
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Or you may want to try a thread with a shared variable, you can pass strings to be eval'd in the thread $data_in. I just used numbers in the simple hack.

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

# declare, share then assign

my $data_in:shared = 0;
my $data_ret:shared = '----';
my $button_control:shared = 0;
my $go_control:shared = 0;
my $die_control:shared = 0;

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

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

    my $label = $mw->Label(
         -width => 50,
         -textvariable => \$val )->pack();
 
    my $button = $mw->Button( -text => 'Start',
                              -command => \&start )->pack();
 
# you need a timer to read the shared var in thread
   my $timer = $mw->repeat(10 , sub{
                  $val = $data_ret;
                });

   my $timer1 = $mw->repeat(10, sub{  if($button_control){
                                         $button ->configure(-text=> "
+Next");
                                         $button_control = 0;
                                         }            
                                   });
                                     
MainLoop;

sub start{
  $button ->configure(-text=> "----");
  $go_control = 1;
  $data_in = int rand(20);  # pass some new data in
}


# no Tk code in thread
sub worker {
   my $count;
    while(1){
      
      if($die_control){
             print "thread finishing\n"; 
             return} 
      
      #wait for $go_control  
      if($go_control){      
          
                 if($die_control){
                    print "thread finishing\n"; 
                           return} 
               
               print "incoming $data_in\n";
               
               $count++;            
               print $count,"\n";
           if($count >= $data_in){ $go_control = 0; $data_ret = $count
+; $button_control = 1;}
           $data_ret = $count;
           select(undef,undef,undef,.25);
         
      
        }else{ 
         $count = 0;
         select(undef,undef,undef,.25); }# sleep until awakened 
   }
    
  return;
}
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