my $AoH = [ sort {
    $a->{under} <=> $b->{undef}
    || $a->{order} <=> $b->{order}
} @$sqldata ];

##</code><code>##

(2 <=> 2) == 0 so
0 || 2 <=> 1 evaluates to 2 <=> 1
(2 <=> 1) == 1 # the answer we wanted

##</code><code>##

select ... order by under, order;