my %datastructure = (
    forward_iteration => {
    },
    reverse_iteration => {
    },
);

##</code><code>##

my %datastructure = {
    elem1 => {
        forward => 1,
        reverse => 2,
    },
    elem2 => {
        forward => 2,
        reverse => 1,
    },
};

##</code><code>##

$datastructures{$elem} = {
    forward => forward_val($elem),
    reverse => reverse_val($elem),
    other_way => other_way_val($elem),
}