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$eventscores = [
 [riderid, actionid, score],
...
 [riderid, actionid, score],
 ...
];

$eventscores = {
  riderid => {
...
  },
  ...
};

foreach my $rider (keys %$eventscores) {
 my @scores = values %{$eventscores->{$rider}};
...

 # do something with $rider and @scores
}

select riderid, AVG(score) 
  from eventscores
 group by riderid
 order by riderid