I meant association in a looser sense than you mention above. For the first set, you're associating passwords with their respective users. In the second set, you're associating the given values with their type ("joe" is the "id" for this user). As for the uniqueness condition, I agree that it's useful, but not a hash's intended use. Incidentally, I think that the following loop uses less memory:

my %unique;
$unique{$_} = undef foreach @list;
@list = keys %unique;
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due to the undef being shared amongst all of the keys of the hash. That may be an urban legend that I heard somewhere, though. ;)