use Tk;

my $mw = new MainWindow();
my $button = $mw->Button(-text => "HEY")->pack();
$mw->after(1,\&thread_sub); # after 1 ms, calls thread_sub
# it'll run like a thread, but isn't really!  
# See http://www.foo.be/docs/tpj/issues/vol1_3/tpj0103-0006.html

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
   $button->configure(-foreground=>'red');
}