Not a compiler option, and it's not "set" to 15. randbits is supposed to show how many bits the c library function used to generate random numbers actually provides. So on WinXP using rand from the msvcrt library, you only get 15 (my recollection was that it was actually 14) bits. Note that this isn't a limitation of Windows;

I stand corrected. Thank you for the clarification. I still think the central concept of my reply does apply. Indeed I had submitted a bug report to AS and never got a reply: always wondered why since it seems so simple to me; now much less so.

if someone were to go to the trouble of bundling a random number generator with perl, there'd be no need to depend on whatever the c library dishes up.

Also, some people have {considered,asked} before (although I can't find fresh references offhand) doing so in order to increase rand's quality too, which is well known not to be particularly good, which is why modules like Math::TrulyRandom exist.