$a <=> $b already forces $a and $b to be interpretted as numbers, always.

It doesn't appear to with perl 5.8.2:

use strict;
use warnings;

my $d1 = '2007030110300020070301133000';
my $d2 = '2007030110300020070301143000';

printf "plain = %d\n", ($d1 <=> $d2);
{
    use bigint;
    printf "bigint = %d\n", ($d1 <=> $d2);
}
{
    use bigint;
    printf "bigint-0 = %d\n", (($d1 -0)<=> ($d2-0));
}
[download]

Output:

plain = 0
bigint = 0
bigint-0 = -1
[download]