You can do this with PL/perlu or PL/pythonu . However, I strongly recommend that you don't do it this way. DNS problems, problems with the server, etc will cause your PostgreSQL backends to stall, severely disrupting database performance and possibly causing you to exhaust max_connections.
Instead, have a trigger send a NOTIFY when the change occurs, and have the trigger insert details into a log table. Have a client application LISTEN for the notification, read the records inserted by the trigger into the log table, and make any appropriate HTTP requests.
The client should get the requests from the log table one-by-one using SELECT ... FOR UPDATE and DELETE it when the request is made successfully.
See this answer about sending email from within the DB for some more detail.