my %AS = (
 requests_id => 'requests.id AS requests_id',
 requests_name => 'requests.name AS requests_name',
 requests_rating => '(select count(*) from ratings where ....) AS requests_rating',
 ...etc...
);

my $sth = $dbh->prepare( qq|SELECT
    $AS{request_id}, $AS{request_name}, $AS{request_rating},
    ...etc
);