Ah yes, I see. In that case, you’re going to have to read through the whole file, and I doubt there’s much you can do to speed up the loop.

BTW, when I saw the regex /123456$/, I assumed you wanted to match 123456 at the end of a line — that’s what the $ anchor means in a regex. If you want to match a literal $, you need to escape it: m{123456\$} or:

use strict;
use warnings;
use autodie;

...

my $password = '123456';

open(FH, '<', "../Tests/10-million-combos.txt");
$counter  = 0;
$counter2 = 0;

while (<FH>)
{
    ++$counter;
    ++$counter2 if /^Q$password/;
}

print "Num. Line : $counter - Occ : $counter2\n";
close FH;
[download]

See quotemeta.

Hope that helps,