#!/usr/bin/perl

# http://perlmonks.org/?node_id=1201500

use strict;
use warnings;

my $file = '/usr/share/dict/words';

open my $fh, '<', $file or die "$! opening $file";
chomp( my @words = grep /^[a-z]{5,}$/, <$fh>);

my @good = grep frequencysequence($_), @words;

print "@good\n\n" . @good . "\n";

sub frequencysequence
  {
  for my $n ( 1 .. ((-1 + sqrt 1 + 8 * length) / 2) =~ s/^(\d+)\..*/$1 + 1/er )
    {
    "@_\n@_" =~ /
      (.).*
      \n
      (?: (?:(?!\1).)* \1 ){$n}
      (?:(?!\1).)*
      $ /x or return 0;
    }
  return 1;
  }