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

In reply to Re: Using the Math::BigFloat->digit() method by Anonymous Monk
in thread Using the Math::BigFloat->digit() method by Athanasius

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