'I think it's good idea to perceive verbs with "по" and without "по" like different verbs.' - Do you mean that the perfective version with "по" can be assumed to have a somewhat different meaning to its imperfective partner?

I understand that English tenses are very different to Russian aspect; I don't see aspect as being about tense, although that is one part of it. There isn't really an equivalent concept to aspect in English, or none that I can find anyway...

I think that maybe a sentence with an additional clause (after the one containing поработал/работал) might have the same meanings? Like 'Я работал до семи' vs. 'Я поработал до семи'.

But are there any simple sentences which use поработал or работал interchangeably that would have the same meaning? Without the added context of more information?