(armor penetration will be referred as "AP")
Okay first of all let me say that i will only talk about the armor penetration part of the items not other passives (even though the 28% of AP are a passive so i'll just take other AP passive into account) cause its always the point ppl think and i just want to teach you all that % doesnt mean late game if its enough or even BETTER than flat AP
i'm talking for the 99.999999% of ppl that goes "why do you go perforator 2nd its for late game cause it's % AP" or "perforator 2nd is bad cause it's % AP flat is better early" you get it
now let me teach you not to have biased opinions about smth just because everyone says so
so if you go for a flat pen item first then a second flat pen second lets say like this
1st : malady (8AP)
2nd Nightfall (12AP)
total of 20 flat AP
and usually you are around lvl 10 when you get your second item if you arent far behind or overfed
most heroes (squishy ones) have around 60 armor lvl 10 (it varies but mostly around 60 lets just round it to have a general and average view and for simplicity sake (cause TB has 57 but sparrow 65 armor)
if you go for those items instead lets say :
1st : Malady (8AP)
2nd : Perforator (6AP flat) +28% (16.8AP)
against a squishy heroe (so the lowest potential value you can get out the 28% AP which stand at approximately 60 armor)
give an equivalent of 16.8 flat AP from passive +6
so total 30.8 flat AP (and more on tanks who have around 70 armor at this point (lvl10))
so the perforator ALONE (but not first) gives 22.8 AP no other AP item give that much even deathstalker (8+10 with fully stacked passive) or painweaver (10+8 with full passive)
and the thing is that the perforators value keeps going up from there
so what i just want to explain you and show you is that
ITS NOT BECAUSE SOMETHING IS EVEN STRONGER LATE GAME THAT IT IS NECESSARILY BAD EARLIER (i said "bad" not "worse" ofc its gonna be "worse" than it later self if it scales later)