while ($str =~ 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
  )                                          # end capture
}xgs) {
  print "run: <$1>\n";
}