First - your connect() call is incorrect. You should have something like:

$dbh = DBI->connect('dbi:Oracle:', $user, $password, $attr);
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where $attr is a hash reference to connection attributes that you want to set (such as RaiseError, PrintError, etc).

Beyond that, however, your code looks OK, so I have to ask: are you sure that there are rows in the table that will satisfy the WHERE condition for both serial_number and item_id?

Michael