# get the customer => blance hash mapping - if you can't RDBMS you are stuck with this
$cust_bal = ......
# now all we are interested in is the top 30
# we don't need to sort them all using a Schwartzian
# we only need to sort the values. This saves us a lot of memory as 
# [ cust, bal ] will take up a lot more space than just bal
# we can also exit at this point and write a new temp flat file
# with all the cust => bal mappings and this reclaims the memory.
# either way we just want those top 30 so just sort the balances:
my @bals = sort { $b <=> $a } values %$cust_bal
# we are only interested in the top 30 which is a balance greater than....
my $gt = $bal[30]
# so now we iterate over our hash again (or the flat file)
my $top_30;
for my $cust( keys %$cust_bal ) {
  next unless $cust_bal > $gt;
    $top_30->{$cust} = $cust_bal->{$cust};
}
# now the top_30 are in a hash ready to be sorted for mapping to cust details and output