In my experience, the next step after a massive hash fails is to go to a database. DBD::SQLite is fast and easy; see Databases made easy for an intro, and http://www.w3schools.com/sql/ for quick reference.

Update: Demo code: 

use strict;
use warnings;
use DBI;
require DBD::SQLite;

unlink 'track.db';
my $db = DBI->connect(  'dbi:SQLite:dbname=track.db', '', '',
                        {
                            RaiseError  => 1,
                            AutoCommit  => 1,
                        },
                    );

$db->do(<<EOSQL);

CREATE TABLE tracking
(
    fname   varchar(255),
    fext    varchar(255)
)

EOSQL

my $count_query = $db->prepare(<<EOSQL);

SELECT  COUNT(*)
FROM    tracking
WHERE   fname=?
  AND   fext=?
    
EOSQL

my $insert_query = $db->prepare(<<EOSQL);

INSERT INTO tracking (fname, fext)
VALUES(?,?)
    
EOSQL

open(my $fh, "<", "input.txt") 
    or die "cannot open < input.txt: $!";
while (my $line = <$fh>) {
    chomp $line;
    my ($fname, $fext) = split(' ',$line);
    $count_query->execute($fname, $fext);
    my ($count) = $count_query->fetchrow_array;
    $count_query->finish;
    if (!$count) {
        print "$fname $fext\n";
        $insert_query->execute($fname, $fext);
    }
}
$db->disconnect;

[download]

Yeah, it's longer, but you are going to have to do work on disk if you can't hold it in memory. Feel like implementing your own Merge sort instead?

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In reply to Re: Large file, multi dimensional hash - out of memory by kennethk
in thread Large file, multi dimensional hash - out of memory by Anonymous Monk

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