It's pretty simple. I don't know how it's documented, since I learned it mostly by folklore (and by watching the response to bug reports on P5P {grin}).

When a variable containing undef (or a new element of an array or hash) is used in an "lvalue-context" (on the left side of an assignment, for example) as a reference, a reference to the appropriate anonymous thingy is inserted and the operation continued, rather than throwing an error for an attempt to dereference undef.

So, push @$x, 35 works even if $x is undef, because we're about to dereference $x while trying to store something.

It's really just an extension of the principle that 

$last_name{"fred"} = "flintstone";

[download]
works, even though I didn't originally create the element with that key.

-- Randal L. Schwartz, Perl hacker

In reply to Autovivification by merlyn
in thread A Not so Simple Append by Limo

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