SQLite/Perl question

I know that SQLite locks the database exclusively when a write is performed on it.  But is there any way of forcing a lock on it?  For example I want to select some rows, then update those rows.  My concern is that in between the time the rows are selected and when I update the rows that something will get written to the database and make my updates inaccurate.

I would love to have a stored procedure to do this to make it all easy but SQLite does not provide that.  So is there any way of forcing a lock on the database BEFORE I do the select and then unlock it AFTER the update?

In perl :)