Laurent_R is correct, but you can get close(r) to what you want with a better rounding function. Consider:

#! perl
use strict;
use warnings;

my @fractions   = (3 / 11, 2 / 11, 1 / 11, 5 / 11);
my @rounded_old = map { round_old($_) } @fractions;
my @rounded_new = map { round_new($_) } @fractions;
my $sum_old     = 0;
   $sum_old    += $_ for @rounded_old;
my $sum_new     = 0;
   $sum_new    += $_ for @rounded_new;

printf "\nOld sum: %.2f new sum: %.2f\n", $sum_old, $sum_new;

sub round_old
{
    return 100 * sprintf("%.2f", shift);
}

sub round_new
{
    return sprintf("%.2f", int((1e5 * shift) + 0.5) / 1e3);
}
[download]

Output:

22:10 >perl 688_SoPW.pl

Old sum: 99.00 new sum: 99.99

22:10 >
[download]

Hope that helps,