Reservoir Sampling in Perl

A method for randomly selecting N values from a finite (but unknown length) list of elements, without replacement. Algorithm is from Taming Uncertainty. This version takes lines of input on STDIN (or files on @ARGV) and selects 20 of them at random and prints the results to STDOUT.

#!/usr/bin/perl

use strict;
my $N = 20;
my $k;
my @r;

while(<>) {
  if(++$k <= $N) {
    push @r, $_;
  } elsif(rand(1) <= ($N/$k)) {
    $r[rand(@r)] = $_;
  }
}

print @r;