my $href1 = $dbh->selectall_hashref($sql1 "key");

my $href2 = $dbh2->selectall_hashref($sql2, "key2");

my $href3 = $dbh3->selectall_hashref($sql3, "key3");

foreach (sort(keys(%$href1))) {
   if ((exists($href2->{$_}{key2})) {
     $sql = "UPDATE data_set2";
     $sql .= "set data = $href1->{$_}{data} where key2 = $_";
   }
   else {
     $sql = "insert into data_set2 $href1->{$_}{data}";
   }
   if ((exists($href3->{$_}{key3})) {
     $sql = "UPDATE data_set3";
     $sql .= "set data = $href1->{$_}{data} where key3 = $_";
   }
   else {
     $sql = "insert data_set3 $href1->{$_}{data}";
   }
}