use strict;
use warnings;
use Tkx;
use threads;

# ... set up your controls

$bar->configure(
  -orient => "horizontal",
  -mode => "indeterminate",
);
$bar->start();
my $thread = threads->create(sub { system("my.bat"); });
$thread->set_thread_exit_only(1);
while ($thread->is_running()) {	Tkx::update(); }
$progress_bar->stop();