From perldoc -f select:

This calls the select(2) system call with the bit masks specified...
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I bet that this tries to redirect you to the system select manpage (man 2 select), so you'll find the documentation there.

Note that whether "select" gets restarted after signals (say, SIGALRM)
+ is implementation-dependent.
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When a signal arrive, it's likely that the select() function exits with "-1" (error), and in C you'd peruse errno to see if it's set to EINTR. I guess that in Perl a good look to $! will give you the same information.

Also, I always thought that sysread() was non-blocking, but when there is no data to read, sysread() blocks.

AFAIK, read() and similars have always been blocking, and you have to explicitly set unblocking mode to get what you want (try to see something in Fcntl and related documentation). Unluckly my mileage in Perl is quite short in these arguments, but I'd stick to select() even if i were programming in C. Given the fact that interruptions result in EINTR, you can easily restart the wait process.

Regarding the last question, I would not expect your program to bomb out if you pass a bigger buffer: it would be quite strange if such a behaviour would not be documented. The maximum (or minimum :) I can expect is that you won't get 100k of data at one time and no more, but I haven't tried it.