Lext01, the identifier $readn is what you're looking for to return the line number you read.
Code:
alias example {
  var %rline = $readn
  if ($read(file.txt,w,$+(*,$$1-,*))) {
    write -dl $+ %rline file.txt
  }
}
This is just a rough example...

As 5618 said, you may have missed the text file name.