What business can we do with a 7 perch land?

Here's the acre to perch to toise conversion:

1 acre = 100 perch
1 perch = 11 toise

7 perch = 77 toise (this is what you have)

Of course you can cultivate sugarcane if the land is fertile but the question you need to ask yourself is whether this is going to be profitable which I don't think it will because you don't have enough land. Even planters with 10 acres land want to convert their agricultural land to residential because that's more profitable.

So residential is better for sure and like Khairah said, you can build a house on that land and rent it out. In the long term, that's going to pay off. However you need to consider getting a residential permit as I'm assuming your land is agricultural from what you said.

Depending on location, you can also turn it into a commercial property. Maybe even have the ground floor commercial and upper floors residential. Or build a whole office on the land.

You can also breed chickens/goats/cows or plant vegetables/flowers on the land. Or rent it to someone who would want to do that.

I've seen people turn their land into a dog breeding place too. Careful if you have neighbours as they won't like the noise though.

There are loads of things to do - sit down, have a think about it all and find something you'd be comfortable with :)