Yes, in this case the goal is on how easy it is to add a progress indicator to an existing code that uses for(). If a user has to change her for() to while(), that would count as less easy. It's akin in spirit to data dumping modules like XXX or Data::Dmp that returns its original argument so you can insert XXX or dmp just about anywhere in an existing Perl code.
Next step after I conquer for(), will probably move on to while(). :-) Yes, I'll be looking into overloading the diamond operator or tying filehandle.