Take a look at https://git.launchpad.net/unrarextractrecover/diff/script/gui.pl?id=b326ba775652a0e320eacb005ee9b2e5fbd95db4

for a complete application/example that updates the progress bar through a thread queue

The relevant code is along those lines:

while (Tk::MainWindow->Count) {
           if (my $queue_message=$main::worker_to_boss_queue->dequeue_
+nb) {
               my ($message,$no)=@$queue_message;   
                     
                  given ($message) {        
            
                        when ("allfiles") {
                            $gui::percent_done=0;
                            $gui::_progress->configure(-to => $no);
                        }
               
                        when ("update") {
                            $gui::percent_done += 1;
                        }
               
                        when ("end") {
                            $gui::percent_done += 1;
                            enable_buttons();            
                        }
                        
                }
        
           }
        
        DoOneEvent(ALL_EVENTS);
        
        }
}
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