With paper and ballot box, you need to trust the people doing the counting.
And since you usually need a lot of them when it's done manually, it's very probable that there's always some amount of fraud.
Now, it's probably easier to fraud if you can hack the voting machines. But let's not say that the current system really prevents fraud because it doesn't.
Hence my argument about large scale fraud (only possible with electronics) vs. small scale fraud. Fraud with paper and ballot boxes is extremely hard to organize at large, because it would involve a couple of orders of magnitude more people to conspire.
And since you usually need a lot of them when it's done manually, it's very probable that there's always some amount of fraud.
Now, it's probably easier to fraud if you can hack the voting machines. But let's not say that the current system really prevents fraud because it doesn't.