@a = (['a',1],['a',2],['b',3],['b',4]); # "db read"
my %aByGroup;

foreach(@a) {
  # you could just push $_->[1], but I'm guessing your
  # data is more complex, so I'm keeping the whole row
  push @{$aByGroup{$_->[0]}},$_;
}

my $old;
foreach my $group (sort keys %aByGroup) {
    my $total = 0;                          # init
    print "Section $group\n";

    foreach my $row(@{$aByGroup{$group}}) {
        $total += $row->[1];            # processrow block
        print "$row->[1]\n";        
    }

    print "$group: $total\n";           # finish row block
}