The diamond operator reads from ARGV, not STDIN.

That's not quite true

choroba is correct though, in that <> is always equivalent to <ARGV>, as is described in the very documentation you referenced. The magical behavior you describe is not that of the <> operator in general, it is that of the ARGV filehandle, which modifies @ARGV in order for STDIN to be read automatically via the ARGV handle.

Reworded for clarity, shortly after posting.