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

# some preparation that requires a database lock

my $firsttime = 1;
foreach my $id (@list_o_object_ids) {
   lock_db_tables() unless $firsttime;
   $firsttime = 0;

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

   # make changes, save back to db.

   unlock_tables();
}