When requesting a result as json the server does not specify a complete Content-Type header:
[ Content-Type: application/json ]

Since the content is UTF-8 the header should be:
[ Content-Type: application/json; charset=utf-8 ]

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$ lwp-request -e -d  "http://validator.w3.org/nu/?doc=http%3A%2F%2Fwww.w3.org&out=json"
Cache-Control: no-cache
Connection: close
Date: Tue, 10 Sep 2013 12:26:18 GMT
Accept-Encoding: gzip
Server: Jetty(6.1.26)
Content-Type: application/json
Expires: Thu, 01 Jan 1970 00:00:00 GMT
Access-Control-Allow-Origin: *
Client-Date: Tue, 10 Sep 2013 12:26:17 GMT
Client-Peer: 128.30.52.91:80
Client-Response-Num: 1
Client-Transfer-Encoding: chunked