I'd like to address the assertion that for forward iterating you need one hash and for reverse iteration, you need another. This seems to me like you would have a hash of hashes, built as such:

my %datastructure = (
    forward_iteration => {
    },
    reverse_iteration => {
    },
);
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This seems wasteful to me. Instead, why not have something like

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

This way, if you want to change your code to add a new ordering mechanism (which happens more often that I'd like), you just add another value to each element's hashref of orderings. And, building the ordered-hash is more obvious, because you're doing something like

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

Just a thought. :-)

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