After upgrading to curl 7.35.0 and rebuilding transmission torrent server and client, the client could no longer connect to localhost:9091, it now always results in a connection refused. The only way I could get it to connect was to remove one of the following lines from my /etc/hosts:
::1 localhost fe80::1%lo0 localhost
But those are standard mappings, so removing them could result in more widespread issues. I was instead able to work arround the issue by having transmission connect to 127.0.0.1 instead of localhost.
I ran into another issue that is probably related, when trying to install a new version of a perl module that wraps curl (LWP::Protocol::Net::Curl). The module includes a test to verify error-handling by trying to connect to a non-existent server at 127.0.0.1:0, but curl is somehow instead connecting to my local webserver on port 80. Even with the standard curl client. No other http client I tried experiences this problem, the others all result in the expected "can't connect" error.
$ curl http://127.0.0.1:0/ <html><body><h1>It works!</h1></body></html> $ curl http://127.0.0.1:1337/ curl: (7) Failed to connect to 127.0.0.1 port 1337: Connection refused
Neither of these problems existed with 7.34.0. I'm running OS X 10.9, curl @7.35.0_0+ssl is installed from Macports.
$ curl -V
curl 7.35.0 (x86_64-apple-darwin13.0.0) libcurl/7.35.0 OpenSSL/1.0.1f zlib/1.2.8 libidn/1.26
Protocols: dict file ftp ftps gopher http https imap imaps pop3 pop3s rtsp smtp smtps telnet tftp
Features: IDN IPv6 Largefile NTLM NTLM_WB SSL libz TLS-SRP