One way is to use any of the SQL::Statement DBDS - AnyData, CSV, DBM, etc. with the new IMPORT() function which allows heterogeneous joins across DBMS types. For example:

my $csv_dbh = DBI->connect( 'dbi:CSV...);
my $ora_dbh = DBI->connect( 'dbi:Oracle...);
my $fir_dbh = DBI->connect( 'dbi:Firebird...);
my $ora_sth = $ora_dbh->prepare(...);
my $fir_sth = $fir_dbh->prepare(...);
my $csv_sth = $csv_dbh->prepare("
        SELECT $cols
          FROM IMPORT(?) AS T1
  NATURAL JOIN IMPORT(?) AS T2 ...
");
$csv_sth->execute( $fir_sth, $ora_sth );
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The syntax for the join must follow the SQL::Statement rules, while the two statement handles follow the syntax rules of the respective DBMSs they query.