sub button_callback{

   use Proc::ProcessTable;

  # prevent button from working on second click
  $start_button->configure(-state => 'disabled');
   
#run your program so it can get its pid , read perldoc perlipc
      my $pid = open(TH, "my_program |");  
 
 # start a timer to check for the $pid 
my $timer;  #declare first so it can be cancelled in its own callback 
$timer = $mw->repeat(1000, sub{
     
    # you can grep a shell and grep thru the output of ps, or              
    my $t = new Proc::ProcessTable;

    foreach my $p (@{$t->table}){
if(  $p->pid == $pid){ print "still running\n";
          }else{   
            #renable button 
            $start_button->configure(-state => 'normal'); }
            
            #cancel the timer
            $timer->cancel; 
       });

}