If you're using a version of Math::BigInt older than version 1.90, you may want to upgrade to the latest. I found this entry in the CHANGES log for the Math::BigInt distribution:

2010-09-03 v1.90 rafl
 * fix bnok() for k==0 and k==n-1
[download]

It looks like the bug-fix in version 1.90 targeted the bug you're experiencing.