Which version of Perl are you using? It works just fine for me on 5.16.1. Using bigint, I get values for every integer. Using bignum, I get values for every int and decimal (except for trailing zeroes). Without an extra module, anything over 20 digits defaults to scientific notation with 15 digits of precision.
in reply to Adding really big numbers
Update: Here is some info on largest positive/negative integers that can be nadled by different setups w/o using modules: Re: maximum value of a scalar