The word “pierogi” is already plural. There is no “pierog” - or rather, there is, but nobody says it. This is the first thing.

The dough

The dough is made from flour, hot water, and occasionally egg. It should be thin enough that you can see your hand through it, but not so thin that it tears when you fold it. The fold matters. The edge should be tight.

The fillings

In the rest of the world, pierogi means potato and cheese. In Poland, it means at least eight different things:

Ruskie - potato and farmer’s cheese. The most common. The name has nothing to do with Russia; it refers to the historical Ruthenian region of Ukraine.
Z mięsem - minced meat and onion.
Z kapustą i grzybami - sauerkraut and mushroom. The Christmas version.
Z jagodami - blueberry. Sweet, eaten with sour cream and sugar.
Z truskawkami - strawberry, in season.

Where the fights are

Krakow people and Warsaw people disagree about the ratio of potato to cheese in ruskie. This is not a metaphor for anything. It is a real disagreement about food.