I'd think it's a hash that looks like a linked list:

%alphabet = (
  a => 'b',
  b => 'c',
  c => 'd',
  # ...
);
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This is because I can start ANYWHERE in the alphabet and continue to the end. But the structure is really a bit more complex than that, since I do know the POSITION of some of the letters, as well as the letter that comes before it. So it's a partially linked list. You'll see my report soon enough...

$_="goto+F.print+chop;\n=yhpaj";F1:eval