Well I don't expect functions whose purpose is to use a system call/function to be completely system-independant. I had in mind the "core language" only, like the original question in this thread.

There's no select in C because it's actually in POSIX/Unix-related libraries, it's not in standard C (C89, C99...) It may be in the same library on your system though, if this library implements both C library and Unix functions.

There are unspecified behaviours in C which really depend on the core language, not the library. For example, in y = f(x) + g(x), you don't know if f is called before g or not, I think it's specified by a compiler, but it's not in the standard. Here I see people say "Try it to see what it's doing", so I suppose in such a case, there's no unspecified behaviour in Perl, because in C testing is not enough to know (f is called before g on Sparc with gcc, g is called before f on x86 with gcc).