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

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

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

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

use Tk;

my $mw = tkinit();
$mw->after(300,\&timestamp);

MainLoop;

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

   $mw->after(300,\&timestamp);
}