The kind of fraud that used to happen in poor areas where a large number of people are illiterate, was that the local Boss would send buses to pick up people and take them to vote, and would provide model ballots with the “right” candidates, to be copied. There was some intimidation involved too. I think this happens a lot less now (at least i haven’t heard about it for years), and the new voting computers show the candidate’s picture onscreen to help people who can’t read.
Mandatory vote means, by the way, that if you haven’t voted in the most recent election, you can’t get a job. Since i live overseas, i don’t actually have to vote, but if i go back to the country to live, i’m supposed to go register again within 3 months of arrival, and explain why i wasn’t able to vote (i.e. prove i was living outside the country).