The simplest mechanism is to just add exit after the MainLoop(). When you exit the gui, MailLoop() returns and the program terminates with cleanup:

sub gui {
    require Tk;

    my $mw = Tk::MainWindow->new;
    
    ...

    Tk::MainLoop();
    exit( 0 );              ### Add this
}
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That is indeed simple. Thank you!

# catch window close button to clean up threads
$mw->protocol('WM_DELETE_WINDOW' => sub { &clean_exit }); 

sub clean_exit{
  my @running_threads = threads->list;
    if (scalar(@running_threads) < 1){print "\nFinished\n";exit}
      else{
        # $die = 1;  # a shared var I usually use
                           # to tell threads to return
       
         foreach my $thr (@running_threads){
              $thr->join;
          }   
      exit;
  }
}
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