Well, a peek into the source as I have it doesn't seem to show that M::BI::N needs any particular version of M::BI and also, in my case:

C:\>perl -MMath::BigInt=9999
Math::BigInt version 9999 required--this is only version 1.87 at C:/Pr
+ogrammi/Pe
rl/lib/Exporter/Heavy.pm line 121.
BEGIN failed--compilation aborted.
[download]

OTOH

C:\>perl -MMath::BigInt -e "Math::BigInt->new(42)"
Math::BigInt::FastCalc is missing method '_from_oct' at -e line 0
Cannot load outdated Math::BigInt::FastCalc v0.10, please upgrade at -
+e line 0
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So I'd better upgrade anyway. This still doesn't explain why the new() method can't be found...

Update: I upgraded Math::BigInt::FastCalc and the problem with M::BI went away. That with M::BI::N stays though.