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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

# 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 members).
for my $gid ( keys %ALL_USER_GROUPS )
   my $members = $ALL_USER_GROUPS{$gid};

   print("Group: $gid\n");

   # Now we want to dump the list of group members.
   for my $member (@$members) {
      print("Member: $member\n");

      # That's it.
   }

   print("\n");
}

##</code><code>##

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