use Tk;
use threads;
use threads::shared;

my $mw = new MainWindow();
my $button = $mw->Button(-text => "HEY")->pack();
my $thread = threads->create(\&thread_sub);

MainLoop();

sub thread_sub
{
   for (my $i=0; $i<10_000_000; $i++)
   {
      # DO NOTHING. LET THE TIME BE TIME
   }

   # CHANGE BUTTON'S TEXT PROPERTY TO "hey" HERE
}