FAQ 4.57 How do I look up a hash element by value?

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4.57: How do I look up a hash element by value?

    Create a reverse hash:

            %by_value = reverse %by_key;
            $key = $by_value;

    That's not particularly efficient. It would be more space-efficient to

            while (($key, $value) = each %by_key) {
                    $by_value = $key;

    If your hash could have repeated values, the methods above will only
    find one of the associated keys. This may or may not worry you. If it
    does worry you, you can always reverse the hash into a hash of arrays

            while (($key, $value) = each %by_key) {
                     push @}, $key;


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