my $sth = $dbh->prepare(
  "SELECT foo_id, col1, col2 FROM a ORDER BY foo_id");
$sth->execute();

my @group;

while (my $row = $sth->fetchrow_hashref) {
  if (!@group or $row->{foo_id} == $group[0]->{foo_id}) {
    push @group, $row;
  } else {
    # print Dumper(\@group);
    process(\@group);
    @group = ($row);
  }
}