I would suggest reading BigInt Math docs in addition to BigInt Main docs. A BigInt is an object.

#!/usr/bin/perl -w
use strict;

use bignum;
use Math::BigInt;

my $i = Math::BigInt->new(4300000000);
print $i->as_bin(), "\n";

#prints 0b100000000010011001100101100000000

my $j = 4300000000;
print $j->as_bin(), "\n";

#also prints 0b100000000010011001100101100000000
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ActiveState Perl 5.10.1 on Win XP 32 bit