This is a pretty cute alternative:

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) a
+nd assigns $var2 as the new value.
empty $var1 = $var2;

# Can also be used like this to assert $var1 is empty without assignin
+g a new value.
empty $var1;
[download]