|laziness, impatience, and hubris|
I have a couple of misgivings about this that go off in a couple of diverging directions.
If what you want to do is just give some sort of entertaining talk, sure. I wonder though if it might also be interesting to discuss and build on some of the existing non-dead-end avenues and might have a more lasting impact outside of a talk.
Installing Devel::REPL from CPAN pulls in lots of stuff including Moose which may be off-putting to some. But on the other hand, it may very well not be a problem for those who are writing Perl using modern OO Perl style. Those people may either already have it installed or want it installed. Some may simply decide to get Moose via their OS distribution mechanism.
But while Devel::REPL pulls in lots of other modules, psh does not.
So the first question: have you have considered contributing to one of these projects instead of going off in yet another direction?
And this leads me to the other aspect of this endeavor I think saddening: yet another (largely dead-end) extension of perl5db. perl5db is what I'd use as a negative advertisement for the Perl language.
Have you considered extending Devel::Trepan for a command-line debugger (which seems to be what you want here) or Devel::hdb for an HTML REST service debugger? Both of these are much more modular and come with tests.
And for having expressions evaluated, have you seen my wiki entry on how expressions are handled in Devel::Trepan?
Good luck with your talk.