You'll probably just want to build your hash as a circular linked list: 
my $position_of = (
      'A' =>  {
               next_key => 'A#',
               prev_key => 'G#',
               #  other stuff . . .
              },
      'A#' => {
               next_key => 'B',
               prev_key => 'A',
               #  other stuff . . .
              },
       ### in-between stuff
      'G#' => {
               next_key => 'A',
               prev_key => 'G',
               #  other stuff . . .
              },
);

[download]
Update: with circular linked lists you usually keep track of head/tail: 
my $head = 'A';
my $tail = 'G#';

[download]

--Jim

In reply to Re: Bijective data association structure? by jlongino
in thread Bijective data association structure? by FoxtrotUniform

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