Another way:

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

use Tk;

my $mw = new MainWindow;
my $time = time();
my $label = $mw->Label(-textvariable => \$time);
   $label->grid(-row => 1, -column => 1);

$mw->repeat(1000, sub{ $time = time(); } );

MainLoop;
[download]