"compared to having to re-engineer the entire codebase to work with a new framework."

"The various web frameworks are fine for green-field projects but impose a significant burden on the developer when migrating in an existing application like this.

In this use case, mojolicious has you covered, adding one hook allows the existing codebase to work as expected. I've used this in the past without issue.

"I have to say that this has not been my experience."

I've rewritten several legacy CGI::Application codebase with Mojolicious/(or Mojolicious::Lite), the learning curve wasn't steep, I ended up with something faster, fewer lines of code and less cludgy than before. Of course YMMY

Update: Also, for the uninitiated, mod_perl binds you to using Apache.