The bug doesn't happen because perl has STDIN line-buffered. Select won't notice STDIN has read data until you press enter. This also means there will always be a newline to read for <STDIN>.

Line-buffering only makes sense for output handles, and STDIN is not an output handle.

Secondly, you don't only use readline (<>) on STDIN.

See Re^3: Malfunctioning select() call on a FIFO for a demonstration of the problem.

It is my hypothesis that the buffer you speak of happens at the system level and not inside the perl interpreter.

Sometimes, yes. Not always.

I have no idea how this works in windows.

Not very well, since select only works on sockets in Windows.