My answer is, essentially, "what ysth said".

The purpose of a hash is to provide fast access to a specific key, rather to store information in a sequential order.

You can always store the order an item was entered, and scan for it later. That is:

$in{$key}{rank} = $nthElement++;

Followed by

foreach $key (sort(keys %in)) {
    print MAIL "\t", $key, " = \t", $in{$key}, "\n"
            if $in{$key}{rank} == 0;
}
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