Current Perl documentation can be found at

Here is our local, out-dated (pre-5.6) version:

Create a reverse hash:

    %by_value = reverse %by_key;
    $key = $by_value{$value};

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

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