I know there are some great modules on CPAN that implement this sort of dual interface.
Which do you have in mind? The only one I can think of is CGI, and it's either the worst example of a dual interface or the best, in that it's slow, complicated, potentially very buggy, and difficult to maintain.
For me the important question is whether an operation needs to encapsulate some kind of state. If so, I use an object interface. If not, I use a procedural interface.
I want to avoid using these until I have a solid grasp of how to implement OO in Perl by hand.
That sounds backwards to me. I've written my own regular expression engine, but I wouldn't suggest someone take a class on automata before performing simple matches.