Well, the use of lexical variables or --in other words-- the my operator is something you should look into very quickly. It makes for very clean programming, allowing you to use variables in sub-routines and loops without having them effect anything outside of the sub-routine or loop. Read the Private Variables via my() chapter in perldoc perlsub if you want to know more.

That being said, you can drop all the my keywords in my program and if you also delete the use strict; it will still run fine, but you really, really should look into the use of my (and use strict;). It wil save you a lot of time and effort over and over again and will guard against stupid and silly mistakes.

The way my program is written, it will always only read one line of the file in memory at any time, so your files can actually have millions of lines and it will still work.