Math::BigInt:

sub blog10 {
    my $bn = shift;

    require Math::BigInt;
    return (ref($bn) ? $bn->copy : Math::BigInt->new($bn))->blog(10);
}

my $bn = 1e12;
my $blog10 = blog10($bn);

print "blog10($bn): $blog10\n";
print "mod($blog10,3): ", $blog10 % 3, "\n";
print "int($blog10): ", int($blog10), "\n";
[download]

The output:

blog10(1000000000000): 12
mod(12,3): 0
int(12): 12
[download]

...which should be what you expect it to be.

This does silently upgrade your number to a Math::BigInt object. Calling ref($blog10) would return Math::BigInt, for example. If that's objectionable, modify your blog10 function as follows:

return (ref($bn) ? $bn->copy : Math::BigInt->new($bn))->blog(10)->numi
+fy;
[download]

You could make this even more "black box" by auto-decorating and auto-undecorating based on logic:

sub blog10 {
    my $n = shift;

    my $bn = ref($n) ? $n->copy
                     : do {
                         require Math::BigInt;
                         Math::BigInt->new($n)
                     };

    my $ret = $bn->blog(10);

    return ref($n) ? $ret : $ret->numify;
}
[download]