Any nonexistent variable, or a variable containing undef, which we dereference while looking for a variable location(technically called an lvalue context), is automatically stuffed with appropriate reference to an empty item

That refers to

$ perl -le'print $r||"[undef]"; $r->{x}; print $r||"[undef]";'
[undef]
HASH(0x814ec28)
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jethro was refering to a different kind of autovivification. Creating an alias (as undef does) to a non-existent array or hash element can create (vivify) the element in question (with value undef).

$ perl -le'sub f {} print 0+keys %h; for ($h{x}) {} print 0+keys %h;'
0
1
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