Maybe this will help, simple example:

use Tk;
use Tk::Button;
use Tk::Label;
use Tk::Text;

my $foo = 0;
my $break = 0;

my $mw  = tkinit;
my $lbl1 = $mw->Label(-textvariable, \$foo)->pack();
my $txt1 = $mw->Text(-height, 1 )->pack();
my $btn1 = $mw->Button( -text, 'Start', -command, \&foo )->pack;

MainLoop;

#sub below the main

sub foo {
  $btn1->configure(-text, 'Type quit in the text box to stop!');
  while(1){
    $foo++;
    $break = $txt1->get('0.0', 'end');
    last if $break =~ /quit/ig;
    $mw->update;
  }
  
  $btn1->configure(-text, 'Start');

}
[download]

JamesNC