I don't think json support has ever been implemented for the web service proper. It's not mentioned on http://musicbrainz.org/doc/XML_Web_Service/Version_2
If you want/need JSON, you can go direct via the search server; however I imagine there is some stuff that can't be retrieved via this mechanism as it will presumably limit you to information held within the search index (not the database); so I believe only queries will be supported - no entity browsing or direct entity retrieval.
For your search, if you want "starts with" you need to use a *. I'm a bit confused as I'd have thought you need artist:"caro e"* to escape the space and avoid it treating it as a separate token - but that doesn't work. Usually a space in the query is treated as an implicit OR. Anyway, no matter, maybe I'm confused :p
Via Search (JSON) http://search.musicbrainz.org/ws/2/arti … p;fmt=json
Via WS (XML): http://musicbrainz.org/ws/2/artist/?query=caro%20e*
(the artist: is implicit since you're already doing a /artist/ search)