if($username eq $row[0] && $hashPassword eq $row[1]) {
     # Go to the new page
  }
  else {
    error();
  }

##</code><code>##

my @rows = $sth->fetchrow_array;

if (scalar @rows == 0) {
  # Unknown user
  error();
}
if (scalar @rows > 1) {
  # DB in bad state - more than one record for user
  some_other_error();
}
if ($hashPassword ne $rows[1]) {
  # wrong password
  error();
}

# ...show the page...