This does not seem to be an average :) First item is divided by 1, second divided by 2, third by 3... - maybe it's better to make the division at the end:

my %oc;
for (0 .. $#sequence) {
  $oc{$sequence[$_]}[0] += $numbers[$_];
  ++$oc{$sequence[$_]}[1];
}
foreach my $seq (keys %oc) {
  print "The average for $seq is ".($oc{$seq}[0] / $oc{$seq}[1]).$/;
}
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