Why do you want to make life so miserable for yourself? Why not ask the database to do the update for you in one hit.

update table1 t1
set t1.field1 = 999
where exists (
    select 1 from table2 t2 where t1.id = t2.id
)
[download]

Then you just have to debug your SQL. No Perl needed apart from a $db->do(''), no bugs to look for! Not to mention faster...