You were talking about system dependencies:

Well since perl is written in C, there could be unspecified behaviours in Perl itself (ie. having different valid results for the same code with different platforms),

Platform and system means almost the same here, doesn't it?

(and it doesn't depend on C at all, there's no select in C).

What do you mean here "not in C"? Select is a system call, so what it does with the time is an issue of the OS kernel, but the restarting of system calls after signals is handled by the C library IIRC. (Or doesn't it, does the kernel restart syscalls if you say so with sigaction, maybe libc just sets the default behaviour to what you want, I'm not sure).