use Math::BigInt;
$d = Math::BigInt->new(0);
foreach my $h (split'','0x0081def509be'){
#UPDATE: Doh! Thinking base 10, this now
#        agrees with IO's result. It also
#        handles arbitrarily large hex values.
#   $d = Math::BigInt->new($d->bmul(10));
    $d = Math::BigInt->new($d->bmul(16));
    $d = Math::BigInt->new($d->badd(hex($h)));
    printf " :: %i : $d \n", hex($h);
}
print $d, "\n";