Take a look at the pg_server_prepare attribute in the DBD::Pg docs.

That feature prevents you from running multiple statements in one prepare. Something that you shouldnt do anyway, since by nature prepared statements are to be reused

On another note, why bother locking?

update: What I mean is, you're just going to have to change your code. It was bad that DBD::Pg let you do this in the old version.