$ cat a
one
two
three
four
five
six
$ (for x in {1..33333}; do perl -we 'my $k = int(shift); my @a; my $l = 0; while (<>) { if ($l++ < $k) { push @a, $_ } else { if ((my $j = rand($l)) < $k) { $a[$j] = $_; } } } print @a;' 3 a | sort | tr \\n \  ; echo; done) | sort | uniq -c | sort -rn
   1747 five four three 
   1736 five one six 
   1735 five three two 
   1725 four three two 
   1707 one six three 
   1695 five four two 
   1685 five six three 
   1684 five six two 
   1678 one three two 
   1666 five four six 
   1663 four six two 
   1663 four one six 
   1663 five four one 
   1645 four one two 
   1640 four six three 
   1637 five one three 
   1616 five one two 
   1592 six three two 
   1578 one six two 
   1578 four one three 
$