Yes, I think this is a bigint bug:

#! perl
use strict;
use warnings;
use bigint;

my $m = '10';
my $k =  '2';
 ++$k;
print $m / $k,    "\n";
print $k->sign(), "\n";
[download]

Output:

13:45 >perl 992_SoPW.pl
3.33333333333333
Can't locate object method "sign" via package "3" (perhaps you forgot 
+to load "3"?) at 992_SoPW.pl line 10.

13:45 >
[download]

This shows that ++ is failing to convert $k into a BigInt object. But change the line ++$k; to $k += 1; and the output is now as expected:

13:45 >perl 992_SoPW.pl
3
+

13:46 >
[download]

The documentation for bigint claims:

All operators (including basic math operations) except the range operator .. are overloaded.

But this doesn’t appear to be true for autoincrement.

Hope that helps,