It's probably faster to iterate instead of sorting: 
sub lowest_key (\%) {
    my @keys = keys %{ shift() };
    my $low = $keys[0];
    foreach(@keys) {
        $low = $_ if $_ < $low;
    }
    return $low;
}

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In reply to Re: Returning the lowest key in a hash (or highest) by MrNobo1024
in thread Returning the lowest key in a hash (or highest) by deprecated

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