This is quick perlish pseudo code for the process just explained in my post just above: 
my %seen;
open my $FH, "<", "file1.txt" or die "cannot open ...";
while (<$FH>) {
    chomp;
    $seen($_) = 1;
}
close $FH;
for my $other_file (qw/ file2.txt file3.txt file4.txt .../) {
    open my $FH, "<", $other_file or die "cannot open ...";
    while (<$FH>) {
        chomp;
        if (exists $seen{$_}) {
            $seen{$_}++;
        }
    }
    close $FH;
}

[download]
Only the first file is ever loaded into memory. When you read the other files, you just update counters. Even if you have hundreds of files, it will work provided the first file can be loaded into the hash.

Update:: moved the second chomp line to the right place (within the while loop, not just before).

In reply to Re^2: write to Disk instead of RAM without using modules by Laurent_R
in thread write to Disk instead of RAM without using modules by Anonymous Monk

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