This set of modules grew out of discussion on the CGI::Application mailing list.
There have been a number of times throughout CGI::Application's lifespan where mailing list members have asked for the ability to inject code inbetween different parts of the request-response process provided by the module. This has led to a proliferation of hard-coded, slightly awkwardly named phases which users can override in their subclasses. But whether you use them or not, all of the phases are always there. And there will likely come a time when someone wants yet another insertion point. This is the first issue that Application::Pipeline tries to address.
A lot of people have also sought either a) alternatives to the convenience methods provided by CGI::Application, or b) want to glue their own favorite Way To Do It more closely to CGI::Application. Right now, CGI.pm and HTML::Template have somewhat priveledged status by being integrated into the core of CGI::Application. Certainly it was not a bad decision at the time of C::A's first writing, but it now has a broad enough user base that there are quite a few of us using alternatives to these modules. Jesse has provided ways of working around this integration, but ultimately it is still a work around. This is the second issue that Application::Pipeline tries to address.
Docs, Source Code, Example Script
I am at the stage where I have working code, and a lot of documentation written. I have not yet gotten to writing tests. What I am looking for right now is constructive criticism on the API, how things could be done better internally, and any other comments that might make this module more worthy of posting to CPAN.