This is straightforward to predict when you separate what's happening on the right side from what's happening on the left.

The list of 

(1 => 2, 3 => $x{1} )

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Is identical to 
(1, 2, 3, undef)

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as long as %x didn't have a previous value.

In fact, it's the prior value of $x{1} that you see, as evidenced by: 

%x{1} = "old x sub 1";
%x = 
  ( 1 => 2, 
    3 => $x{1} 
  );

[download]

The question for me is, why did you find this surprising or baffling? It's extremely consistent with what Perl does at all times: evaluate the right side of the assignment before altering any structure on the left. It's what permits 

@a = (2, @a)

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or 
@a = reverse sort @a;

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to work.

-- Randal L. Schwartz, Perl hacker

In reply to Re: late binding behavior in hash definition by merlyn
in thread late binding behavior in hash definition by princepawn

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