# 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");
}