Super Search throws up lots of results about Tk threading issues, and I guess How to change a Tk object's property from a thread will tell you what you need to know.

how can i implement it without using threads (using fork) ?

One detail you did not mention. Is your background process contained in another script which you can call, or is it in a block of code?

Here is a simple fork example to get you started, otherwise show some code. Also the Tk::ActivityBar module needs to be installed manually, it is used only for a quick demonstration, but it is a problem-prone module.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::ActivityBar;

my $mw = MainWindow->new;

my $activity = $mw->ActivityBar(-anchor => 'w')->pack(-expand => 1);

$mw->Button(-text => 'Start Activity',
   -command => sub{ 
              $activity->startActivity(); #start the bar
              my $timer = $mw->after(6500,
                sub{ $activity->configure('-value' => 0); } );
              &do_it;
          })->pack();

$mw->Button(-text => 'Exit',
    -command =>sub{ exit })->pack();

MainLoop();

sub do_it{
  # you need to fork your command or it will block the gui
  # from functioning

  if(fork == 0){
    system("date; sleep 2; date; sleep 2; date; sleep 2");
  }
}
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