lock_db_tables(); # lock tables and/or start a transaction 
                  # in a database independent way

# 1100 lines of preparation that requires a database lock
# must be in same transaction as first iteration of loop
# below

foreach my $id (@list_o_object_ids) {

   my $obj = new Thingy($id); # gets stuff from db

   # make changes, save back to db. (about 900 lines in my case)

   unlock_tables();
} join {
   lock_db_tables();
}