$x= "0x0081def509be";
use Math::BigInt;
sub bighex{
     my $x = shift;
     return Math::BigInt->new(hex($x)) if 8 > length $x;
     return (bighex(substr $x,0,-4)<<16) + hex substr($x,-4);
}

$d = bighex($x);
print "$d\n";