It is true that one will probably immediately reassign the $- variables. And that's definitely true that there are cases when conventions impede good style. I would probably say, though, that in most cases the latter of your examples is preferable.

    if string =~ /First: (.*?) Last: (.*?)\s/
      first_name = $1
      last_name = $2
      # etc...
    end

looks far better to me than

    if string =~ /First: (?<first_name>.*?) Last: (?<last_name>.*?)\s/
      # etc...
    end