#!/usr/bin/env perl

my $k = shift @ARGV;
srand;
while (<>) {
    $. <= $k and do { push @lines, $_; next; };
    ($a=rand($.)) < $k and ($lines[$a] = $_);
}
print @lines;

##</code><code>##

$ seq 4 20 | ./rand.pl 8
19
5
6
14
16
9
15
11