Autovivification saves a lot of annoying effort. So if $quick is originally empty

$quick->{brown}->{fox} ->{$jumped} = 1 / $Lazy_dog;
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creates the intermediate hashes so $quick can point to a hash-of-hashes-of-hashes.

But autovivification can bite you / surprise you, too. If you are testing a value (rather than setting it) autovivification can still add entries to your hash. That is, new keys. E.g., if you start with

$stateCityPopulation =
 {Illinois => {Springfield => 116482},
  Massachusets => {Springfield => 154082},
  Missouri   => {Springfield => 150797}}};
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and then you

 
foreach my $state (qw(Illinois Massachusets Ohio Missouri)){
  my $pop = $stateCityPopulation->{$state}->{Springfield} || next;
  $springfielders += $pop}
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Now when you look at keys %$stateCityPopulation, 'Ohio' is in there. It's holding an empty array. It was created when you were trying to access the value for $stateCityPopulation->{Ohio}->{Springfield}. Even though you got undef back as a value, the itermediate array got created 'by side effect.'

If you're debugging code, and you find an array has a bunch of unexpected keys, which hold empty hashes, then autovivification is to blame. If you'd changed the code in the above to

$pop = ($stateCityPopulation->{$state} && 
        $stateCityPopulation->{$state}->{Springfield}) || 
        next;
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You'd have skipped the autovivification (at the expense of messier code.)

BTW, if you're debugging a rogue autovivification, the 'w' switch in the debugger is very handy.

throop