How about:
SELECT SUBDATE(datefieldname, INTERVAL 13 MONTH);

You can then format the result as needed with DATE_FORMAT(). I'm willing to bet both processes will be more efficient done through database than Perl.

EDIT: Oops - as pointed out by grinder in PM, he already specified that the database approach wouldn't work, since the field is TEXT. Somehow I missed that. Whoever designed the table needs some serious punishment.