Math::BigFloat 1.998
Math::BigInt 1.9991
Math::BigInt::Calc 1.997

##</code><code>##

sub _len {
    my $cx = $_[1];
    (@$cx-1)*$BASE_LEN+length(int($cx->[-1]));  # <--- line 1209
}

##</code><code>##

sub _digit {
    my ($c,$x,$n) = @_;
    my $len = _len('',$x);
    ...
}

##</code><code>##

@ISA = qw/Math::BigInt/;

##</code><code>##

...
my $CALC = 'Math::BigInt::Calc';
...
sub digit {
    my ($self,$x,$n) = ref($_[0]) ? (undef,@_) : objectify(1,@_);
    $n = $n->numify() if ref($n);
    $CALC->_digit($x->{value},$n||0);
}

##</code><code>##

my ($self,$x,$n) = (undef, $YOUR_n, -1);

##</code><code>##

Math::BigInt::Calc->_digit(undef, -1);

##</code><code>##

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

use Math::BigFloat;

my $n = Math::BigFloat->new(123);

say $n;
say 'REF $n: ', ref $n;

eval {
    say $n->digit(-1);
};
warn $@ if $@;

say '$n->{value} EXISTS: ', exists $n->{value} ? 'Yes' : 'No';
say '$n->{value} DEFINED: ', defined $n->{value} ? 'Yes' : 'No';

eval {
    $n->{value} = [123];
    say '$n->digit(-1): ', $n->digit(-1);
    say '$n->digit(1):  ', $n->digit(1);
    say '$n->digit():   ', $n->digit();
};
warn $@ if $@;

##</code><code>##

123
REF $n: Math::BigFloat
Can't use an undefined value as an ARRAY reference at /Users/ken/perl5/perlbrew/perls/perl-5.18.1t/lib/5.18.1/Math/BigInt/Calc.pm line 1209.
$n->{value} EXISTS: No
$n->{value} DEFINED: No
$n->digit(-1): 1
$n->digit(1):  2
$n->digit():   3