If *ARGV is already open, then the next <> would continue reading from it before looking for the next file name in @ARGV. So you probably used <> before you called the subroutine that you described.

You could close *ARGV, but that would disrupt the other code that was already using <> to read from STDIN. So you might instead want to do local( *ARGV ) in your subroutine before you assign to @ARGV.