There are many forms of "fixed length records". Binary, ascii,, mixed. But assuming you just want to space out the fields to fixed widths, you need to make two passes.

The first finds the maximum lengths of each field, by splitting using split or one of the csv modules as appropriate, and using length (and List::Util::max).

The second uses the maximum field widths from the first pass to format the data for output using either printf or pack.

You'll then need to decide whether each field should be left or right justified and what your overall record length should be and so on. And most of that will depend upon what you intend to do with the fixed width record file afterwards. Is it intended for use by another application? If so, what are the formatting requirements of that application?

There is enough there to get you started, but a clearer picture of what you are trying to do would help a lot.