A simple way, that blocks the gui, but I show for educational purposes:

sub button_callback{

$startbutton->configure(-state=>'disabled');
system( "your_program" );
$startbutton->configure(-state=>'normal');
}
[download]

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; 
       });

}
[download]

There are some gotchas to watch out for, like making sure the $pid is not confused with the program's shell's pid.... you can also use threads. Which is better, depends on how your system program runs. You may even be able to read it's STDOUT or STDERR and watch a filehandle until you see it's closed.

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