The most simple solution for you is to open the file and read into a variable, modify your line(s), and rewrite the file.
Some sample code:

use CGI;

$q = new CGI;
$upd_id = $q->param('id');
$upd_title = $q->param('title');
$upd_description = $q->param('description');

open F, "file.txt" or die "Can't open file.txt: $!\n";
while (<F>) {
  chomp;
  ($id, $title, $description) = split /\|/;
  if ($id == $upd_id) {
    ($title, $description) = ($upd_title, $upd_description);
    $changed = 1;
  }
  $upd_file .= "$id|$title|$description\n";
}
close F;

if ($changed) {
  open U, ">file.txt" or die "Can't open file.txt: $!\n";
  print U $upd_file;
  close U;
}
[download]

I assumed the user submits the information in separate fields. (they used to in real life:)
BTW, besides using huge files which can slow this script dramatically, I advise start getting closer to Mysql, or any similar relational database.
If more user is assumed to access this application, also consider using file locking.

-- tune