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

##</code><code>##

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

##</code><code>##

foreach my $rider (keys %$eventscores) {
 my @scores = values %{$eventscores->{$rider}};
 # or, if you need the scores in order
 my @scores = map {$eventscores->{$rider}{$_}} sort keys %{$eventscores->{$rider}};

 # do something with $rider and @scores
}

##</code><code>##

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