I am probably getting in trouble here, being only a scribe, but I would like to express my feelings on these very worthy questions.
I end up using OO-Perl:
When I find that the structures that I am using in my program have inherent subroutines (methods) associated with them, or when the structures build on substructures in a logical way.
I also have found that I will rewrite some procedural hack as a module (OO prefered here) when the hack looks like it might actually be useful to several applications or to other folks.
Question 2: I do whole lot of precedural hacks, maybe 1-100 lines of code. I wouldn't use objects on these, unless, of course, I am using a Perl Module that lends itself to an object/method model. (Most do!)