I've got an additional question:

Before posting, I had read the man pages of setuid() and its colleagues, and I guess I have understood them.

However, I couldn't find a statement anywhere about what function Perl actually uses to perform assignments to $(E)[U|G]ID. perlvar only tells us that it uses a syscall, which made me believe that it is not one of the setuid() functions. The latter are not syscalls; they are from the C library (please correct me if I am wrong).

But from your post I got the impression that Perl uses indeed the setuid() functions to perform assignments to the user and group variables. Did I understand this correctly?