I guess it depends on the country. In Poland my experience is that every time EU passes some regulation Polish parliament passes the corresponding bill implementing it. So even if we left EU tomorrow those bills will still be in effect.
Also, I'm not sure about this "immediately enforceable" part - I recall some cases where member states delayed implementing EU laws for years, sometimes ending up being sued to European Court of Justice.
Also, I'm not sure about this "immediately enforceable" part - I recall some cases where member states delayed implementing EU laws for years, sometimes ending up being sued to European Court of Justice.