#!/usr/bin/perl -w
#
use strict;

my $logfile = "log.file";
my @time = localtime();
my $time = $time[2] . ":" . $time[1] . ":" . $time[0];

{
  open IN, "<", "$logfile";
    chomp(my@logfile = <IN>);
  close IN;

  if ($logfile[-1] =~ /$time/) {
    exit 1
  }
}

use Tk;

my $mw = tkinit();
$mw->after(100,\&timestamp);

MainLoop;

sub timestamp {
  open OUT, ">>", "$logfile";
    print OUT "$time\n";
  close OUT;

  $mw->after(100,\&timestamp);
}