How do I do a 301 redirect to an exact URI only?

If I understand your requirements, you want all requests to abc.com to be redirected to a single alternative URL regardless of the request-uri fragment supplied for the original request. That is, http://abc.com/place/any/path/here?and=any&query=parameters would be redirected to the exact URL http://def.com/locations. If so:

when HTTP_REQUEST {
    HTTP::respond 301 Location "http://def.com/locations"
}