my $obj = new My::Custom::Object;
use bigint;
my $mult = 2**100 *  $obj;

##</code><code>##

package Math::BigInt;
use overload '*' =>
sub {
    if (ref $_[1] eq 'My::Custom::Object') {
         $_[1]->MultiplyByInt($_[0]);
    }
    else {
         $_[0]->bmult($_[1]);
    }
}