I think you are correct in pursuing sprintf as one good way to skin this cat. But I must admit, I personally prefer working with delimited records to fixed-length records any day of the week. Are you sure you can't do this with a delimiter, rather than fixed-length?

If you decide you must use a fixed-length strategy, you might try declaring a hash with attribute names and expected string length (or other formatting instructions). Then you could pass each form input to a standard write_me() routine that would look up the appropriate formatting instruction and write it out to your file (or whatever).

I'm sure there are lots of TIMTOWTDI variations. :)