There's an entry in perlfaq5, How do I change one line in a file/delete a line in a file/insert a line in the middle of a file/append to the beginning of a file?. The current answer is quite boring:

Use the Tie::File module, which is included in the standard distribution since Perl 5.8.0.

I like the old answer (perl 5.6.x) much better — and much more fitting for solving a homework problem:

Those are operations of a text editor. Perl is not a text editor. Perl is a programming language. You have to decompose the problem into low-level calls to read, write, open, close, and seek.

Although humans have an easy time thinking of a text file as being a sequence of lines that operates much like a stack of playing cards -- or punch cards -- computers usually see the text file as a sequence of bytes. In general, there's no direct way for Perl to seek to a particular line of a file, insert text into a file, or remove text from a file.

(etc...)