Regarding

{4|8}
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this is a nice idea, but doesn't do anything special. a curly bracket is only a special character when it is found in one of these forms {n}, {n,} or {n,m}.

As your example isn't like this, the bracket is just matched as a plain character. What your first regex shows is just an alternation, equivalent to the following...

if (/[0-9a-f]\{4/ or /8\}/} {
  print "matched\n";
}
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Note that I've escaped the curly bracket just to be explict, it's not actually necessary