sub empty ($) :lvalue {
	if ($_[0]) {
		require Carp;
		if (eval { require PadWalker }) {
			my $varname = PadWalker::var_name(1, \$_[0]);
			Carp::croak("$varname is not empty, stopped");
		}
		Carp::croak("Variable is not empty, stopped");
	}
	$_[0];
}

# Asserts that $var1 is "empty" (i.e. the empty string, 0, or undef) and assigns $var2 as the new value.
empty $var1 = $var2;

# Can also be used like this to assert $var1 is empty without assigning a new value.
empty $var1;