$balances{$cust} += $bal * $fx_rates->{$ccy};

##</code><code>##

  my @sorted_cust_list = 
    reverse map {$_->[0]} sort {$a->[1]<=>$b->[1]}
    map{[$_,$balances{$_}]}keys %balances;

##</code><code>##

  while ( my($cust,$balance) = each %balances ) {
      ...
  }