OK, you've been shown what to do.

I was going to suggest that perhaps reading some of the very fine manual would help you... however, when I looked for @ARGV in the online documentation it doesn't exactly leap out at you... but see @ARGV in perlvar. It's an array like any other.

The <> construct in your fragment is a peice of magic to work through (what remains of) @ARGV treating each item in turn as a file name, opening and reading.