# return 1 (true) if user is found, 0 (false) if not found
sub check_exist {
  $username = shift;

  # do the necessary dbi conection...

  $sql = qq{SELECT count(*) FROM members WHERE nick=?};
  $sth = $dbh->prepare($sql);
  $sth->execute($username) or bail_out("Can not execute SQL query");
  my ($rowcount) = $sth->fetchrow();
  $dbh->disconnect();

  return $rowcount > 0;
}

##</code><code>##

if (check_exist($user)) {
   # handle_exception...
} else {
   # user does not exist
}