(?:                                      # this chunk:
      $                                        # either the end of the string
      | (??{ chr(1 + ord substr($&, -1)) })    # or the next character (ASCII-wise) after
                                               # the one we just matched
    )+                                       # one or more times
   matches null string many times in regex; marked by <-- HERE in m/
  (                                          # capture to $1
    .                                        # any character
    (?:                                      # this chunk:
      $                                        # either the end of the string
      | (??{ chr(1 + ord substr($&, -1)) })    # or the next character (ASCII-wise) after
                                               # the one we just matched
    )+                                       # one or more times
   <-- HERE )                                          # end capture
/ at ./x line 17.
run: <hi>
run: <abc>
run: <bc>
run: <xyz>
run: <ijklmn>