Take the example you provided for instance. That Map can only take a String and Another String.
[snippet]
Map<String, String> phoneBook = new HashMap<String, String>();
phoneBook.put("J. Random Hacker", "555-1337");
[/snippet]

It cannot nest in more Maps because the phone book only accepts String objects. In Perl, you can do some crazy things like ...

[snippet]

%phonebook =
(
'name' =>
{'J. Random Hacker',


);