Sure, that's why I later on say that linebuffered output of less than 512 bytes is safe. I was just explaining the concept and (over)simplifying things a bit.

Notice by the way that when writing STDOUT to a tty perl is linebuffered and print "aaa\nbbb\n" will in fact become two writes, even without setting $|. But the real problem of course is the target program becoming blockbuffered (most likely with a blocksize fitting in a pipe which nowadays usually are 4K), but the block not ending on a lineboundary.