So how can I get this code to read from a .txt file?

You have to write code to open a file, read data from it (perhaps line-by-line with readline), and then operate on the data using techniques shown you by rjt and others and developed by yourself, and then write the transformed data as appropriate.

I'm sorry to be so vague, but this is a very general question! Perhaps something like chromatic's freely-downloadable Modern Perl would help.

So how can I get this code to read from a .txt file?

This thread seems to be turning into a "How do I process a data file?" kind of discussion. If so, it would probably be better to start another thread (perhaps with a link back to this thread for reference) in SoPW with a request of that nature. Be sure to include info on the structure of the data in the file you wish to process.