$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";