It's not; autovivification is defined in perlref and is perl automatically treating undef in an lvalue reference context as evaluating to a reference to a newly created item of the correct type (arrayref or hashref). This is at best just undefined in a numeric context (in this case coming from an empty slot in a hash) being treated as a numeric value of zero.

(Now were it $words{$something}->{$_}++ and $words{$something} wasn't defined for a given value of $something then it WOULD be an instance of autovivification (in which case a new hashref would be conjured up, stuck into $words{$something}, and then the key $_ in that new hashref would be looked up (and since it's an empty hash would evaluate to undef which would numerically be zero then that would be incremented to one, and finally that one stored back into that hashref)))

(But as usual for the source . . . INEW)

Edit: tweaked first paragraph to be even more technically correct (the best kind).

The cake is a lie.
The cake is a lie.
The cake is a lie.

In reply to Re^4: regex, find words that occur more than once. by Fletch
in thread regex, find words that occur more than once. by Anonymous Monk

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