The error is not in the subroutine.

In your script when you call create_output     (@array_of_lines, $entry_no_new); the array is empty since you never put anything into @array_of_lines.

You are getting yourself confused because in the look_through_file subroutine you also declare an @array_of_lines array and fill it with data. BUT as this is a lexical array scoped to this subroutine only (and totally separate of the lexical @array_of_lines declared in the main body of your script), it will simply disappear at the end of the subroutine, taking all its data with it.

There are two solutions:

1. use the @array_of_lines declared in the main body of your script also in the subroutine. In other words, do not do a my @array_of_lines in your sub. THIS IS THE BAD SOLUTION!
2. Take the return value from your look_through_file sub and put that in the @array_of_lines array declared in the main body of your script. You will have to re-adjust the return parameters of that sub and put the array at the back or the returned scalar values will get eaten by the array.