The CGI script:

#!/usr/bin/perl -Tw
use strict;
use CGI;
use CGI::Carp 'fatalsToBrowser';
use Fcntl ':flock';
use POSIX qw(strftime);

my $now = strftime "%b %e", localtime;
my $q = new CGI;
my $user = length $q->param('user') ? $q->param('user') : "Anonymous";
$user = $q->escapeHTML($user);
my $message = $q->param('comment');

if ($message){ # Avoid posting blank messages
  open FH, "+</var/www/html/comment.txt" or die "Oops: $!";
  flock (FH,LOCK_EX) or die "Couldn't flock: $!";
  my  @comments = <FH>;
  seek (FH ,0,0);
  truncate (FH,0) or die "No can do: $!";
  print FH "<br><b>On $now, $user added this bit o' wisdom:</b><br>$message<br><hr>\n";
  print FH @comments;
  close FH;
}

print $q->redirect('http://server.com/index.shtml');