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    <p>It is specifically guaranteed that this is undependable.</p> <p>See the <a href="http://perldoc.perl.org/perlsec.html#Algorithmic-Complexity-Attacks" rel="nofollow">Algorithmic Complexity Attacks</a> section of <code>perlsec</code> in full. Despite its regrettable incoherency, it states that</p> <ul> <li>in 5.8.1, the order is guaranteed to be random every time.</li> <li>in 5.8.2 and later, the order will be the same unless Perl detects pathological behavior (specifically, a series of keys that would all hash to a small number of buckets, causing the hash performance to suffer). In those cases, "the function is perturbed by a pseudorandom seed".</li> </ul> <p>The documentation does <em>not</em> guarantee that the ordering will always be the same; in fact, it specifically states that it will <em>not</em> be predictable in a pathological case. Should the hashing function be changed in a future release, it's possible that data which previously did not generate degenerate hashing would now do so, and then would be subject to the random perturbation.</p> <p>So the takeaway is that if you're not using 5.8.1, <em>maybe</em> you'll get the same order, and <em>maybe</em> it won't change when you update your Perl, but it might. If you are using 5.8.1, then a random order is <em>guaranteed</em>.</p> <p>If you want a dependable order, use one of the CPAN classes that provides a hash that has a guaranteed key order - <a href="http://p3rl.org/Tie%3a%3aHash%3a%3aIndexed" rel="nofollow">Tie::Hash::Indexed</a>, <a href="http://p3rl.org/Tie%3a%3aIxHash" rel="nofollow">Tie::IxHash</a> - or just sort your keys. If you have a hash that has less than a few thousand keys, you probably won't notice an appreciable difference. If it has more than that, maybe you should consider a heavier-weight solution such as a database anyway.</p> <p>Edit: and just to make it more interesting, <a href="http://www.perlmonks.org/?node_id=1005122" rel="nofollow">keys will be randomly ordered as of 5.18</a>. </p>
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