Sorry, I'm new to this and I don't have too much time to read the tutorials, but this I still don't understand $seen{$_} I know $_ is the current stream. .= is like adding it at the end. But what is that hash /10$/ ? But this only matches the exact line length it doesn't look for a the same word. what if I want to find a word in both files and print it out on screen? Thanks.