Since your code requires to reference previous hash values from new hash entries, you should populate the hash one entry at a time, instead of sending in the entire list. This gives you the most freedom to intertwine the references, spot recursions and order the assignments according to your underlying logic. That is:

my %h = ();
$h{'word1'} = [2, 3];
$h{'word2'} = [\${'word1'}, 3];
$h{'word3'} = [1, 2];
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Given the way you wrote the assignments, you suggested that you expect every hash entry to be assigned one at a time, which is exactly what the above piece of code does. The reasons why Perl does not assign them the way you'd expect them to is first because it treats the hash entries as a regular list containing a key, a value, another key, another value and so on, and secondly because it calculates the values of every element in this list (key or value) before assigning it to a hash variable. The reason why you got a scalar reference is because Perl treats \undef as a reference to a scalar, a behavior which you cannot avoid even setting warnings and strict on.