#!/usr/bin/perl
use strict;
use Tk;
BEGIN {*Tk::Clock::time = sub { time * -1 } }
use Tk::Clock;
my $wm = MainWindow->new;
my $ck = $wm->Clock->pack();
$ck->config(dateFormat => "dd-mm-yyy");

MainLoop;
[download]

This clock displays the time since the Epoch - but into the past.

To have it display the time going back from the moment the clock started, use

BEGIN {
    my $time = time;
    *Tk::Clock::time = sub { 2 * $time - time };
}
[download]

before loading Tk::Clock.

update: while being silly, it also shows an interesting fact about UNIX time, and how to override core functions for modules to-be-loaded... eeevil... ;-)

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}