This function works like the system call by the same name: it makes the named directory the new root directory for all further pathnames that begin with a / by your process and all its children. (It doesn't change your current working directory, which is unaffected.) For security reasons, this call is restricted to the superuser. If FILENAME is omitted, does a chroot to $_.
NOTE: It is good security practice to do chdir("/") (chdir to the root directory) immediately after a chroot.
Portability issues: "chroot" in perlport.