sub vcl_hash{
  set req.http.X-Hash = req.http.host;
  ... more stuff ...
  set req.http.X-Hash = req.http.X-Hash "#"; # add trailing '#' to be compatible to varnish 2.0
  set req.hash += req.http.X-Hash; # store in unreadable req.hash
  return(hash);
}

sub vcl_deliver {
  ... more stuff ...
  # store hash in response for matching / testing that hashing works
  set resp.http.X-Hash = req.http.X-Hash;
  return (deliver);
}

varnishadm -T 127.0.0.1:6082 'purge req.http.X-Hash ~ something'