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my %quick_allow = map { $_ => 1 } @allow_users;

my @anon;
for (@allow_users) {
   push @anon, do { $_ => 1 };
}
my %quick_allow = @anon;

my %quick_allow;
for (@allow_users) {
   $quick_allow{$_} = 1;
}

my %ALL_USER_GROUPS = (
                    23 => [ qw( g1  g4 ) ],
                    13 => [ qw( g3  g5 ) ],
                    );

my %ALL_USER_GROUPS = (
                  # key   value
...
                    '23', [ 'g1', 'g4' ],
                    '13', [ 'g3', 'g5' ],
                    );

# At the outmost level, you have a list of groups stored in a hash.
# List the keys (group ids) and fetch the corresponding value (group m
+embers).
...

   print("\n");
}

for my $gid ( keys %ALL_USER_GROUPS )
   my $members = $ALL_USER_GROUPS{$gid};
...
   print("Members: ", join(' ', @$members), "\n");
   print("\n");
}