You need to understand what the diamond operator does:

• Does the operator have an active filehandle, if not, shift an argument off @ARGV and open it for input as the active filehandle or if @ARGV is empty, set the active filehandle to STDIN.
• If called in list context, return all lines from the active filehandle.
• If called in scalar context and the filehandle is at the end of the file, return undef.
• Otherwise, return a single line from the filehandle.

That's pretty much it. I might be forgetting some subtleties.

The key bit, I've highlighted for you in bold. It uses @ARGV to figure out what file to read, and it alters that array!

So the first time you read a file using the diamond operator, you read it fine. The second time you try to read the file, the filename is no longer on @ARGV, so the diamond operator reads from STDIN instead.