I'll leave the reading of the hash as an exercise to the user, but here's a fun way to populate the hash:

Update: Apparently, here's a really slow, dangerous, stupid, fraught with peril, performancing dehancing way to do it. Oh well, some code just sucks, don't it?

#!/usr/local/bin/perl -w

use strict;
use Data::Dumper;

{
   my %hash = ();

   while (<DATA>)
   {
      my $s;

      $s .= "{'$_'}" foreach (/(...)/g);

      eval ('$hash ' . $s . '++');

      die $@ if $@;
   }

   print Dumper ([\%hash]);
}

__DATA__
aaabbbcccddd
aab
aacbbb
aaabbacca
aaabbaccb
ababbbddd
[download]

---

$VAR1 = [
          {
            'aab' => 1,
            'aba' => {
                       'bbb' => {
                                  'ddd' => 1
                                }
                     },
            'aac' => {
                       'bbb' => 1
                     },
            'aaa' => {
                       'bba' => {
                                  'cca' => 1,
                                  'ccb' => 1
                                },
                       'bbb' => {
                                  'ccc' => {
                                             'ddd' => 1
                                           }
                                }
                     }
          }
        ];
[download]

--Chris

e-mail jcwren