One method would be to create an app-specific lockfile before starting an app and not start the app again until the lockfile was removed. The app has to delete the lockfile on finish. Disadvantage: If the app crashes without deleting the file the app is blocked

A better method would remember the process id of the app and check if it still exists before starting it again.

And if you want to be 100% sure you could use interprocess communication to ask the app if it still exists. But that would be overkill in most situations

sub Tk_loop { 
    Tk->tkwait('variable',\$giveup); 
    $count_DoOne++, Tk::DoOneEvent(0) until $giveup; 
    $count_loop++; 
    $giveup = 0; 
}
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sub button_callback{

$startbutton->configure(-state=>'disabled');
system( "your_program" );
$startbutton->configure(-state=>'normal');
}
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That way is bad programming, except for emergencies. :-)

Another way, is to disable the Start button until the last run is completed. You can then fork your program off, with a pipe, so you don't have to worry about waiting for it to return. System will interefere with the event-loop of gui's.

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; 
       });

}
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When you launch a process, grey out all your buttons. Then install a signal handler to wait on SIGCHLD that ungreys your buttons.

- doug