I have a large file (30 million records) containing a two-field key that is supposed to be unique. Unfortunately, it isn't necessarily that way... let me try to explain:

1. The file comes to me with these fields (amongst others):
   • Account Number
   • Account Open Date
   • Account Close Date
2. Account numbers can be reused, but only if the account has been closed. Unless you have data issues. Which I do. :(
3. So I need a quick way to check while processing (amongst up to 30 million unique keys) whether I have seen this key before (i.e. whether it's a dupe that needs checking).
4. The normal hash lookup method $cache{$account} works, but with ever-decreasing performance and an ever-increasing memory profile (it got up to 1.4GB of memory usage).
5. So a Perl monger suggested looking at Bloom filters. More on Bloom filters is available here: Perl.com

I've found the Bloom::Filter module in CPAN but can't get it to work and am also worried about what level of false-positives I'm facing.

My current code is:

my $bloom_filter = Bloom::Filter->new(error_rate => 0.001, capacity =>
+ 30000000);

if ($bloom_filter->check($account_number)) {
     ... do deduping ...
} else {
     $bloom_filter->add($account_number);
     ... do something ...
}
[download]

I'm looking for wisdom on two fronts:

• My current code just warns of a lack of salts for the filter, but I can't determine for the life of me what this module is looking for in terms of a salt as the documentation is... minimal... and everything I've tried adding causes my script to malfunction in spectacular ways
• The level of false positive matches is inverseley proportional to the memory footprint of the filter but I am having trouble with the calculations that would enable me to locate the right tradeoff.

Thanks