Another idea might be to use a hash to translate these. Something like this:

  $line = 'foo ?&k2S bar ?&13A baz';

  %cvt = (
    'k2S' => 'AAA',
    '13A' => 'BBB',
  );

  $line =~ s/\?\&(...)/$cvt{$1}/g;

  # prints "foo AAA bar BBB baz"

  print $line, "\n";
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If there's a way of decoding these symbols, you could write a separate routine to create the hash first.