That's a good question. It's because at this point I'm still learning how to write my own middleware. I don't think the Perl Advent Calendar requires the use of those modules if one is merely writing one's own middleware. :) I'd appreciate your further comment. Thank you.

Um, read http://advent.plackperl.org/2009/12/day-23-write-your-own-middleware.html

Thank you for suggesting this link. I copied a part of the Perl Advent tutorial below,

"PSGI middleware behaves like a normal PSGI application but wraps the original PSGI application…”

Based on this, a middleware behaves like a normal PSGI application. This means that I could also treat a middleware like a regular PSGI application and map a URL to it, using Plack::App::URLMap, just like any regular PSGI application. I guess, what I did above to run my middleware $form_validator is acceptable.