Not to be picky about it, but don't even use sleep to demonstrate a slowdown..... in a GUI it completely gives you bad behavior.

Here is one way to make a non-blocking delay without sleep

#!/usr/bin/perl
use warnings;
use strict;
use Tk;

$|=1;

my $count = 0;
my $loop  = 0;
my $mw    = new MainWindow();

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

$mw->Button( -text => 'Start', -command => \&long_job )
  ->pack( -side => 'left' );

$mw->Button( -text => 'Stop', -command => sub { $loop = 0; print "$cou
+nt\n"; } )
  ->pack( -side => 'left' );

MainLoop();

sub long_job {
my $var;
my $timer = $mw->repeat(1000, sub{$var++} ); 

   $loop =1;       
   while($loop){
          $loop++;
          $mw->waitVariable(\$var);
          $count++;
         }
  DoOneEvent();  
}
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