use strict;
use Tk;

my $counter = 0;
my $mw = MainWindow->new(-title => "Counter");

$mw->Label(-textvariable => \&runCounter)->pack;

$mw->Button(
   -text => 'Start Counter',
   -command => \&counter
)->pack;

MainLoop;

sub runCounter {
   $counter = 0;
   while ($counter != 100000) {
      $counter++;
   }
}