Unless I'm mistaken, "DBIx::" is merely a prefix for "things that bolt-on to DBI". It's not one package. You can't "learn" "DBIx" any more than you can learn the first half of a phone number in any useful way.
After reading your reply I went back to the DBI website and saw that
it has a link to the DBIx modules under the title "DBI Extensions", so
clearly all of the DBIx modules are things that bolt onto DBI. That
tells me that I can start by reading the DBI book.