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;