select() only blocks until the filehandle is ready (from performing an implicit autoflush). Therefore, once this has happened the first time, nothing has changed in your loop to prevent the filehandle being ready instantly the second time around, so the blocking occurs thereafter for zero time.

Update: I note that in the code the filehandle stays set to empty, which would default it to standard input - this could be creating an additional obstacle..

Hope this helps,

Simon.