I should have read more before posting. Searching for bignum on metacpan.org did not return the bignum module but on search.cpan.org it does and it says:
The entire upgrading/downgrading is still experimental and might not work as you expect or may even have bugs. You might get errors like this:
Can't use an undefined value as an ARRAY reference at
/usr/local/lib/perl5/5.8.0/Math/BigInt/Calc.pm line 864
This means somewhere a routine got a BigFloat/Lite but expected a BigInt (or vice versa) and the upgrade/downgrad path was missing. This is a bug, please report it so that we can fix it.