my $message = join '', map cleanline($_), <DATA>;
  ...
  {
    my $prevline;
    sub cleanline {
      my $line = shift;
      # skip duplicate lines
      return if defined($prevline) && $line eq $prevline;
      $prevline = $line;
      # crudely HTMLify
      return "$line\n<br>\n";
    }
  }