Or one line: my @row = $dbh->selectrow_array($sql,{},$key);

Never noticed that before on the $dbh object. But more of those types of things are useful.

my $data = $dbh->selectrow_array( "SELECT $cols FROM IMPORT(?) JOIN IMPORT(?) ON(...", {}, $postgresql_sth, $mysql_sth );

I kinda like that. The funky IMPORT(?) doesn't seem standard, but something more "SELECT * FROM $dbh1 A, $dbh2 B where b.something = a.something" seems more natural. In some databases, I frequently do, "

SELECT 
  * 
FROM 
  ( SELECT 
    * 
  FROM 
    A 
  WHERE 
    A.something = 1 ) AA, 
  ( SELECT 
    * 
  FROM 
    B 
  WHERE B.something = 2) BB 
WHERE 
  AA.bob = BB.bob
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nitpick pick pick :)