The standard hash does not maintain any order, if you want some order being stored, for example the order pairs being added, then you have to do this your self (or use some other modules).

If you prefer existing modules, check out Tie::IxHash.

If you want to do it yourself, here is some OO solution I had, which I called it OrderedHash. The code given below is strictly for the purpose to explain the structure more clearly, not any kind of actual implementation.

This OrderedHash structure contains two parts:

1. An array, normal array, which contains the key pair values of the OrderedHash. In the Array, those key-value pairs are stored in this sequence: k1, v1, k2, v3, k3, v3...
2. A hash, normal hash, its keys are the keys of the OrderedHash, values are indexes point to the key of the actual key value pairs in the above array.

A brief analysis of this structure:

1. It reserves the order.
2. It is independent, yet not much implementing effort
3. We don't invent or implement any hashing algorithm, we simply use a normal hash for indexing.
4. Its performance is the same as a normal hash. We don't sacrifice performance.
5. This solution DO sacrifice space to gain performance and to reduce implementation effort, it uses as twice as many space used by a normal hash that has the same number of elements.

The following code is only to demo the structure.

package OrderedHash;
use strict;

sub new {
   my $self = {};
   $self->{INDEX} = {};
   $self->{PAIRS} = [];
   bless $self;
   return $self;
}

sub set {
   my ($self, $key, $value) = @_;
   push @{$self->{PAIRS}}, $key;
   $self->{INDEX}->{$key} = $#{$self->{PAIRS}};
   push @{$self->{PAIRS}}, $value;
}

sub get {
   my ($self, $key) = @_;
   return $self->{PAIRS}->[$self->{INDEX}->{$key} + 1];
}

sub list_in_order {
   my $self = shift;
   my $is_key = 1;
   foreach (@{$self->{PAIRS}}) {
      if (!$is_key) {
          print "$_\n";
      } else {
          print "[$_] = ";
      }
      $is_key = !$is_key;
   }
}

1;

use OrderedHash;
use Data::Dumper;
use strict;

my $oh = new OrderedHash;
$oh->set("a", 1);
$oh->set("b", 2);
$oh->set("c", 3);
print Dumper($oh);
$oh->list_in_order;
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