The official libcurl solution to abort a running HTTP request is to return a non-zero value from the onProgress callback (http://curl.haxx.se/libcurl/c/curl_easy_setopt.html#CURLOPTPROGRESSFUNCTION).
The problem is that std.net.curl will throw an unspecific CurlException, so the API offers no way to handle that error.

----
import std.net.curl;

void main()
{
    auto http = HTTP("www.example.com");
    http.onProgress = (dltot, dlnow, ultot, ulnow)
    {
        if (dlnow > 1024)
            return 1;
        return 0;
    };
    http.perform();
}
----
std.net.curl.CurlException@std/net/curl.d(3592): Operation was aborted by an application callback on handle 1C94B00
----

I think allowing to pass a boolean to perform() so it returns the error code instead of throwing an error would be a good solution. Note that this is already possible for Curl.perform but not with HTTP.perform, FTP.perform or SMTP.perform.

http://dlang.org/phobos/std_net_curl.html#.Curl.perform
http://dlang.org/phobos/std_net_curl.html#.HTTP.perform
http://dlang.org/phobos/std_net_curl.html#.FTP.perform
http://dlang.org/phobos/std_net_curl.html#.SMTP.perform