Learn Hardcore Russian: Грузчик поднял тяжёлую коробку и поставил её у двери. - The mover lifted a heavy box and put it by the door.

Грузчик is in the nominative case because it’s the subject (the person doing the actions). It’s a masculine noun meaning something like a porter / mover / loader (someone who carries and loads goods).

Why does the verb end in -л: поднял, поставил?

That -л marks the past tense in Russian. In the past tense, the verb also shows gender/number:

поднял, поставил = past, masculine singular (because грузчик is masculine singular)
If the subject were feminine: подняла, поставила
Plural: подняли, поставили

Are поднял and поставил perfective or imperfective, and why?

Both are perfective: поднял (from поднять) and поставил (from поставить). Perfective is used here because the actions are presented as completed, single events: he lifted it (done) and set it down/placed it (done).
Imperfective versions would be поднимал / ставил, which would typically suggest process, repetition, or background action.