#this is psuedo-codeish
use Tk;
my $mw = new Tk::MainWindow;
my $p_bar = $mw->ProgressBar;
# No longer needed:
# my $button = $mw->Button(-command=>\&main_logic, -text=>'start');
# $button->pack();

$p_bar->pack();

#this is the solution, thanks to Ven'Tatsu:
$mw->after(0,\&main_logic);

MainLoop;

sub main_logic {    
   # $button->configure(-state=>"disabled");
   for (...) {
      # do some stuff...
      $p_bar->value($pct_complete);
      $mw->update();
   }
   $mw->destroy;
}