As I now understand it, it's *all* of the other variables (not just 'pos') that should be declared in PREINIT instead of CODE.

C doesn't allow intermixing of vars declarations and code like Perl and C++. Var declarations can just be placed globally or at the start of the block. PREINIT will be placed before any code. CODE can be preceded by other code. Therefore, declarations must be placed in PREINIT or you must add curlies to CODE.

The PREINIT section of perlxs explains this in more detail and gives example.