$sql = "SELECT col_2 FROM theTable WHERE col_1 = $value_1";
$sth = $dbh->prepare($sql) or die("Could not prepare!" . $dbh->errstr);
$sth->execute() or die("Could not execute 1!" . $dbh->errstr);
while ($data = $sth->fetchrow_array()) {
	$union{$data} = undef;
}
$sql = "SELECT col_2 FROM theTable WHERE col_1 = $value_2";
$sth = $dbh->prepare($sql) or die("Could not prepare!" . $dbh->errstr);
$sth->execute() or die("Could not execute 2!" . $dbh->errstr);
while ($data = $sth->fetchrow_array()) {
	$isect{$data} = undef if exists $union{$data};
}

@isect = keys %isect;