Well... I can't say I'm enthusiastic about your approach here. Objects are slow enough without turning every method into a wrapper around another sub call; and I don't see how a proceedural module is any eaisier to write tests for than an object-oriented one (though, to make testing a little eaiser, I'm inclined to design my object classes so you can get a bare-bones object with a simple "new" without arguments).

A typical test for me is something like:

 {
   my $test_name = "rect_probe";
   my $bf = Image::BoxFinder->new();
   my $spot = $bf->rect_probe;
   is_deeply( $spot, [ 0, 0 ], "Testing $test_name: found rectangle");
}
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The extra line to create the object isn't a lot of overhead; and myself I just think of the method call ($bf->) as just a short alias for Image::BoxFinder::.

On the subject of mixing and matching methods and proceedures in one module: this makes some people queasy, and I have mild reservations about it myself, though they're only mild.

On the subject of modules with both an object and a proceedural interface, myself I think they're over-kill. Remember: if you put in two totally different interfaces, you're going to have to document both of them.