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$sql = "SELECT action_id
  FROM transactions LEFT JOIN actions 
      ON transactions.transaction_id = actions.transaction_id
...
$aref = $sth->fetchrow_array;
print join(', ', @$aref),"\n";
$sth->finish;

mysql> SELECT action_id,transaction_date
    ->   FROM transactions LEFT JOIN actions 
    ->       ON transactions.transaction_id = actions.transaction_id
...
|   1061498 | 2014-08-05 18:50:11 |
+-----------+---------------------+
1 row in set (0.00 sec)

$sql = "SELECT action_id
  FROM transactions LEFT JOIN actions 
      ON transactions.transaction_id = actions.transaction_id
...
$sth->execute;
$aref = $sth->fetchrow_array;
print join(', ', @$aref),"\n";