Description:

The select system call (and the IO::Select interface) allows us to
process multiple streams simultaneously, blocking until one or more of
them is ready for reading or writing. Unfortunately, this requires us
to use sysread and syswrite rather than Perl's buffered I/O functions.
In the case of reading, there are two issues with combining select
with readline: (1) select might block but the data we want is already
in Perl's input buffer, ready to be slurped in by readline; and (2)
select might indicate that data is available, but readline will block
because there isn't a full $/-terminated line available.