こんなのはどうでしょう｡
%taikyuu = (
'A' => 10,
'B' => 20,
'C' => 15,
'D' => 12,
'E' => 8);
%fuka = (
'A' => 100,
'B' => 80,
'C' => 60,
'D' => 60,
'E' => 88);
@hashkeys = keys %taikyuu;
@hashkeys = sort @hashkeys;
#重さが10,11,12の荷物が来たとき
foreach $_(10,11,12){
undef $minikey;
undef $minifutan;
foreach $hashkey(@hashkeys){
$futan = $fuka{$hashkey} + $fuka{$hashkey} * $_;
if(!$minifutan or $futan < $minifutan){
$minifutan = $futan;
$minikey = $hashkey;
}
}
$fuka{$minikey} += $taikyuu{$minikey} * $_;
}

自分の勉強がてら書いてみた｡

#!/usr/bin/env perl

package Container;

use Moose;

has _initial_load => (is => 'ro',
 isa => 'Int',
 required => 1);
has _persistence => (is => 'ro',
 isa => 'Int',
 required => 1);
has name => (is => 'ro',
 isa => 'Str',
 required => 1);
has weight => (default => 0,
 is => 'rw',
 isa => 'Int',
 writer => '_weight');

around BUILDARGS => sub {
 my $orig = shift;
 my $class = shift;

 if (@_ == 3) {
 my ($name, $initial_load, $persistence) = @_;
 return $class->$orig(_initial_load => $initial_load,
 _persistence => $persistence,
 name => $name);
 } else {
 return $class->$orig(@_);
 }
};

sub add_weight {
 my $self = shift;
 my $weight = shift or die;

 $self->_weight($self->weight + $weight);
 return $self;
}

sub clone {
 my $self = shift;

 return Container->new(_initial_load => $self->_initial_load,
_persistence => $self->_persistence,
name => $self->name,
weight => $self->weight);
}

sub load {
 my $self = shift;

 return $self->_initial_load + $self->weight * $self->_persistence;
}

package main;

use Modern::Perl;

my $container = [Container->new('A', 10, 100),
 Container->new('B', 20, 80),
 Container->new('C', 15, 60),
 Container->new('D', 12, 60),
 Container->new('E', 8, 88)];

foreach my $weight (10, 11, 12) {
 (sort {$a->clone->add_weight($weight)->load <=>
 $b->clone->add_weight($weight)->load}
 @$container)[0]->add_weight($weight);
}