No really swift code I can think of, which prevents the keys from being inserted. But if you can allow empty keys to exist briefly, you can audit them afterwards:

$a = { d=>1, e=>2, f=>3, g=>undef, h=>0, i=>4 };
delete @$a{ grep { not $a->{$_} } keys %$a };
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Of course, that's just shifting the ugliness.