To me it sounds like a hash would be a perfectly fine representation of a spreadsheet line. If you do it with dynamically created accessors, the program using the spreadsheet-line class would either have to find out which methods are supported, which doesn't offer any advantage over using a plain hash, or it would need to know the names of the columns you'll be using in advance.

If you want to do it the OO way, you could have a method "fields" or so that returns an array of strings, and one "get_field" that you can pass a field name and it retrieves the value. Unless the class does something smarter on top of that though, that's completely identical to what a plain hash does, just slower and more verbose.