I've been reading that. And the example of dropping setuid permissions has utterly baffled me (my comments added):

            my @temp     = ($EUID, $EGID);      # Store EUID and EGID
            my $orig_uid = $UID;                # Store UID and GID
            my $orig_gid = $GID;
            $EUID = $UID;                       # Set EUID and EGID to
+ UID and GID
            $EGID = $GID;             
            # Drop privileges
            $UID  = $orig_uid;                  # Set UID and GID to..
+. themselves?
            $GID  = $orig_gid;
            # Make sure privs are really gone
            ($EUID, $EGID) = @temp;             # Set effective UID an
+d GID what 
                                                # they were before? He
+lp!
            die "Can't drop privileges"
                unless $UID == $EUID  && $GID eq $EGID;    # Test UID 
+and GID
[download]

I don't get that.