What am I doing wrong?

I think you found a bug

$ perl -e " use bignum; my $q = 123.0; print join q{ }, $_, $q->digit(
+-$_),$/ for 1 .. 5 ; warn ref $q "
1 1
2 2
3 3
4 0
5 0
Math::BigInt at -e line 1.
[download]

Right, bug is bug

$ perl -e " use bignum; my $q = 123 + 1/3; print join q{ }, $_, $q->di
+git(-$_),$/ for 1 .. 5 ; warn ref $q "
Can't use an undefined value as an ARRAY reference at ...lib/Math/BigI
+nt/Calc.pm line 1209.
[download]

Not surprising since its not clearly tested in the test suite http://search.cpan.org/grep?cpanid=PJACKLAM&release=Math-BigInt-1.9993&string=%3Edigit&i=1&n=1&C=9