$ perldoc -m Const::Fast | grep SvREADONLY
	if (my $reftype = reftype $_[0] and not blessed($_[0]) and not &Internals::SvREADONLY($_[0])) {
		&Internals::SvREADONLY($_[0], 1);
	Internals::SvREADONLY($_[0], 1);
	croak 'Attempt to reassign a readonly variable' if &Internals::SvREADONLY($_[0]);