$<

• $REAL_USER_ID
• $UID
• $<

  The real uid of this process. You can change both the real uid and the effective uid at the same time by using POSIX::setuid(). Since changes to $< require a system call, check $! after a change attempt to detect any possible errors.

  Mnemonic: it's the uid you came from, if you're running setuid.