Isn't the only way to tell juveniles apart by looking at their teeth?
I used to have a filementosum peru that looked like the 2nd picture, but from all what I know both species are essentially the same in an aquarium unless you can provide an insane amount of space for the true piraibe can grow larger, so I would not worry to much about it. And not all variants grow that large, like the mentioned peru variant which is more commonly sold.