#!/usr/bin/perl

use strict;
use warnings;

my $max     = 6;
my $lines   = 20;
my $prob    = $max / $lines;
my $cnt     = 0;

while(<DATA>) {
    last if $cnt > $max;
    (rand(1) <= $prob) and $cnt++ and print;
}

print "last one was on line: " . $.;

__DATA__
foo
bar
buz
qiz
quz
foo
bar
buz
qiz
quz
foo
bar
buz
qiz
quz
foo
bar
buz
qiz
quz