possible to do a nested query with DBI/oracle ?

Yes. (BTW, a quick google shows me a few examples of oracle FROM subqueries and they all are aliased. I'm not sure if such an alias is necessary (in postgres it is) but it is probably a good habit to get into.)

Basically, it doesn't get to the fetchall_arrayref statement.

How do you know?

$odstthandle = $dbttods->prepare($qry) or die $dbttods->errstr;
$odstthandle->execute() or die $odstthandle->errstr;
$billets = $odstthandle->fetchall_arrayref or die $odstthandle->errstr
+;
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Why are you surprised? There is no output in the code so there will be no output.

Can you provide a better example? An example that actually shows some unexpected output (or even an unexpected lack of output)?