use Benchmark qw(cmpthese);
use Math::BigFloat;

Math::BigFloat->precision(-2);

my $number = "999999999999999999999900";

cmpthese -1, {
    substr   => sub { my $num=$number; substr($num,-2,0) = '.'; }, 
    BigFloat => sub { my $num=$number; Math::BigFloat->new($num)/100; }, 
};

__END__
              Rate BigFloat   substr
BigFloat    3794/s       --    -100%
substr   1668189/s   43874%       --