$x{a} &&= 1; is effectively the same as 

$x{a} = $x{a} && 1;

[download]

which is in this case 

$x{a} = undef && 1;

[download]

which is 

$x{a} = undef;

[download]

In reply to Re: &&= vivify keys? by Eliya
in thread &&= vivify keys? by oha

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