I am creating the hash with the keys only and no values.

That is not possible.    In Perl's hashes, all keys have a value, and conversely, all values have a key.

            $mailhouse{$f4} = "";
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Actually, you are assigning a string as a hash value, and then later on trying to use that string as an array reference which is not possible!    If you had strict enabled then Perl would have informed you of this mistake.

What you should be doing is assigning an anonymous array as your hash value:

            $mailhouse{$f4} = [];
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