CREATE TABLE respondent (
    id unsigned int not null auto_increment primary key
   ,name varchar(20)
#  , etc ...
);

CREATE TABLE response (
    id unsigned int not null auto_increment primary key
   ,name varchar(20)
);

CREATE TABLE responses (
    respondent unsigned int
   ,response   unsigned int
   ,PRIMARY KEY (respondent, response)
);

##</code><code>##

SELECT A.respondent AS respondent
  FROM responses A JOIN responses B on (A.respondent = B.respondent)
 WHERE A.response = ?
   AND B.response = ?

##</code><code>##

my $sth = $dbh->prepare_cached( $sql );
$sth->execute( $response_x, $response_y );

$sth->bind_columns( \my ($respondent) );

my @respondents;
while ($sth->fetch)
{
    push @respondents, $responses;
}

$sth->finish;