Your points are taken and are definitely valid. As for your final question where you asked if $obj->isa( 'Product::Real' ) really saves anything over $obj->is_real, I would argue 'yes'. By not hardcoding class names I'm not constrained to a type. As a general rule, for OO programming, it's more important to rely on an object's cabalities rather than its type. Regrettably, Perl's limited introspection capabilities makes this a bit problematic, but that's a rant for another day :)