in reply to
Modify values of tied, split lines in a file
My very-candid opinion is that you are creating an unholy monster that you will regret for the entire brief remaining tenure of your employment. Don’t tie to a file just to avoid reading the thing line-by-line and using split; or, better, using a CSV-file handling package of known provenance. Don’t try to “cut out seemingly-wasteful steps” only to have the program, for example, crash-and-burn in the middle and in so doing leave your both-input and-output file destroyed.
Step back completely from your present approach and reconsider the whole thing. You are being led-on into unknown territory by the allure of the unfamiliar. There are no words of warning strong enough to use here.