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]

From a design point of view, it is usually not a good idea to mix Tk program with end-less loops. I usually seperate endless processing away into a second script.

So you have:

• a GUI program allows to view progress or whatever.
• a background script does the endless loop.

In case, you think the mixture is not avoidable for whatever reason. Try two thing:

• Do your endless loop in this way: 1) wrap the code inside the endless loop in a function. In this function there is no more endless loop, but only one iteration. 2) Using Tk::after to schedule the function once. 3) Inside the function, at the end of it, again use Tk::after to schedule itself once. The third step creates a endless loop, but it will not hang you GUI.
• In your endless loop, periodically (could be once each loop) update Tk widget. This will avoid your Tk from hanging. Try this:

  use Tk;
  use Tk::ProgressBar;
  use strict;
  use warnings;
  
  my $count;
  
  my $mw = new MainWindow(-title => "demo");
  $mw->Button(-command => [\&a_long_process, \$count], -text => "A Long 
  +Process")->pack();
  my $pb = $mw->ProgressBar(
          -from => 0,
          -to => 100000,
          -blocks => 100000,
          -colors => [0, 'green', 30000, 'yellow' , 60000, 'red'],
          -variable => \$count
  )->pack();
  MainLoop;
  
  sub a_long_process {
      my $hash = {};
      for (0..100000) {
          $hash->{$_} = $_;
          $count ++;
          $mw->update(); #try to comment and uncomment this
          ;
      }
  }
  [download]