But the perl docs should be clearer about whether sysread will ever read fewer bytes than requested in cases where those bytes *are* available to be read.

First of all, I don't see how the answer could be anything other they yes. There are countless layers involved. Presumably, one of them could always provide more bytes as long as the handle is still open.

In short, why does it matter when more data will come? You've got data to process.