Hi,

update: it's modulus and not modules!

I recommend the modulus trick of course, but, just to prove TIMTOWTDI:

#!/usr/bin/perl

use strict;
use warnings;

my $counter = 1;
for my $i (1..20) {
    # do something with your file, p.e. print $i . " ";
    ++$counter; if ($counter > 5) { print "."; $counter = 1; }
}
[download]

So for twenty lines we print four dots (each fifth line).