If you can rely on the data looking exactly like this every time then you can just hack off the bit that stops it being JSON:
json_data = json.loads('{' + json_file.read().partition('{')[2])
(or use a regex, or whatever you prefer for string manipulation)
You should still beware that JavaScript is not the same as JSON, so you need some confidence that what you're given will always be an assignment of a valid JSON associative array to a variable.
If, on the other hand, you're being passed arbitrary JavaScript and you're expected to evaluate it, then you have a much bigger problem to solve. What if someone gives you var xyz = "Monthly"; var abc = {"label" : xyz, "URL" : "xyz" + "." + "com"}?
You need a JavaScript engine to solve that more general problem, not just a JSON parser. For example, js2py claims to be a complete JavaScript engine written in Python, but I've never used it so I can't recommend or dis-recommend it.
There are also security issues to consider in executing code from untrusted sources, beyond those you consider if all you're doing is parsing JSON from an untrusted source. So if you need to use a JavaScript engine, make sure you properly understand how it is sandboxed.