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

my %datastructure = {
    elem1 => {
...
        reverse => 1,
    },
};

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