It's my understanding that the 'readers': <> and read() are filtered through the C stdio library

I don't think that's correct — at least not for most modern perls that are built to use PerlIO.  As can be shown using ltrace, the PerlIO read/write operations eventually directly map to the system calls read(2) and write(2) (i.e. not going through the respective stdio/libc calls).  In other words, AFAICT, any buffering and newline conversion behavior that applies to stdio is irrelevant here.