#!/usr/bin/perl

# http://perlmonks.org/?node_id=1173445

use strict;
use warnings;

use List::Util qw(sum);

my @count = (0) x 20;

for (1 .. 2e4)
  {
  my ($top, $bottom) = (20, 20 * 20);
  my @orderedRands =
    grep{  $bottom > 0 && rand(1) < $top / $bottom
      ? (--$top, --$bottom, 1)
      : (--$bottom, 0) }
    map{ ($_) x 20 } 1 .. 20 ;
  $count[$_]++ for grep {defined} @orderedRands;
  }

my $sum = sum(@count);
print "$_\t=>\t$count[$_]\t(", sprintf('%.2f', ($count[$_]/$sum)*100), "% TTL)\n" for 1 .. 20;