while (...)
  {
    $balances{$cust} += $bal * $fx_rates->{$ccy};
  }
  ...
  # Find the customers whose total balance is greater than $500000000.00
  my %top_balances;
  my $top_threshold = 510000000.00;   # default threshold $0.5 bil
  my $top_count; # keep a count of how many found

  do {
    # lower threshold by 10 million every time
    $top_threshold -= 10000000.00;
    die ("This is impossible, the data has error!")
        if ($top_threshold < 0.00)
    $top_count = 0;
    while ( my ($cust, $balance) = each %balances )
    {
      if ($balance > $top_threshold)
      {
    	$top_balances{$cust} = $balance;
    	$top_count++;
      }
    }
  } while ($top_count < $nth);
  # Safety check - how big is our list? Is it > n?

  if ($VERBOSE) {
    print "$top_count customers have ",
          "balances greater than  \$${top_threshold}.\n";
  }

  # Sort only the top customers by descending order of balance
  my @sorted_cust_list =
    reverse map { $_->[0] }
    sort { $a->[1] <=> $b->[1] }
    map { [$_, $top_balances{$_}] }
    keys %top_balances;