my %seen;
my $num_unique_users =
    grep !$seen{ $HoH{$_}{user} }++,
    keys(%HOH);

##</code><code>##

my %seen;
my @unique_users =
    grep !$seen{ $HoH{$_}{user} }++,
    keys(%HOH);

##</code><code>##

my %seen;
my @unique_users =
    grep !$seen{$_}++,
    map $HoH{$_}{user},
    keys(%HOH);