Try enclosing the whole structure into an array ref, and don't forget commas!

[
    { "_id" : 0, "name" : "Betty Boop",
             "subjects" : [
                { "type" : "optional", "subject" : "Voice"},
                { "type" : "required", "subject" : "Math" },
                { "type" : "required", "subject" : "English" } ]
    },
    { "_id" : 1, "name" : "Donald Duck",
             "subjects" : [
                { "type" : "optional", "subject" : "Accounting" },
                { "type" : "required", "subject" : "Math" },
                { "type" : "required", "subject" : "English" } ]
    }
]
[download]

By the way, here is a utility i use to help debug JSON files:

alias djson='perl -MData::Dumper -MJSON -MFile::Slurp -e"print Dumper 
+decode_json read_file shift"'
[download]

L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)