Re: varying length multi-dimensional hashes

Since you're dealing with references, you can traverse your hash with a pointer-type of reference.

my %hash;
while (<STDIN>) {
   chomp;
   my $pointer = \%hash;         # start from root
   while (s/^(...)//) {          # keep eating 1st 3
      if ($_) {                  # if there's more to go,
                                 # we go deeper
         $pointer->{$1} = {} unless ref($pointer->{$1});
         $pointer = $pointer->{$1};
      } else {              
         $pointer->{$1}++;       # otherwise, set it to 1
      }
   }
}
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Note that the following data will mess things up slightly:

111222333444555
111222333

$HASH{111}->{222}->{333} == 1  # lost 444 and 555

111222333
111222333444555

$HASH{111}->{222}->{333} != 1  # now a hash reference
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These are unavoidable though with the criteria you've given. This code also assumes that your string will always be of a length that is divisible by 3. If you have two trailing characters at the end, the substitution will fail, but the test for $_ will have succeeded in the previous loop, giving you a dangling empty hash reference instead of 1.

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Think about Loose Coupling
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