It's hard to help without knowing what those XXX and YYY commands are.  So just some general comment: it's not possible to externally influence the bufferering behavior of a program except by providing some execution context that the program itself might test, like interactive vs. non-interactive mode (the former is usually assumed when the std* file handles are connected to a (pseudo-)tty).  Or by using some interface that the program might explicitly provide for that purpose (usually not available).

In other words, if XXX is not flushing its output in time, you'd have to look into why it (XXX, not Net::SSH::Expect) is not doing it...