s/PAT/STR/     <=>  m/PAT/ && $& = "STR";
  s/PAT/CODE/e   <=>  m/PAT/ && $& = do {CODE};
  s/PAT/CODE/ee  <=>  m/PAT/ && $& = eval do {CODE};
  s/PAT/CODE/eee <=>  m/PAT/ && $& = eval eval do {CODE};
  ... and so on