The CPAN module Tie::DxHash will allow you to make a hash that retains the order the keys were entered, and allows duplicate keys as well.

Here's a short example to show you how simple it's usage is:

use Tie::DxHash;

tie my %hash, 'Tie::DxHash';

%hash = (
  a => 1,
  a => 2,
  b => 2,
  b => 3,
);
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An interesting thing to note is that in the assignment to %hash the duplicate keys are not clobbered.

You can then iterate over this hash using while/each or a foreach/keys, and the elements will be returned in the order they were inserted into the hash.