That reads as if it made a call to binmode superfluous

You're wrong. Without binmode the file handle is in text mode. With binmode but without specified layer, or with layer "raw", it is in binary (raw) mode. That's not the same thing at all, at least, on platforms other than Unix.

A call to binmode is not superfluous. Instead, I think forgetting about it is the reason why the OP got corrupted data:

I know I'm fubar'ing the handling somehow; just don't know how :P

p.s. to the OP: hex('0x17') is a silly, slow way to do things, if you can just enter a hexadecimal numeric literal: 0x17.