Can you cite your sources? Where did you learn about autovivification? in perl?

You offered these two snippets but they're not perl :)

$ perl -le " my $bar[396]++;  "
syntax error at -e line 1, near "$bar["
Execution of -e aborted due to compilation errors.

$ perl -le " my $bar[396]; "
syntax error at -e line 1, near "$bar["
Execution of -e aborted due to compilation errors.
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$foo is a named variable, it has a name, it doesn't need to be autovified, it exists by virtue of being named, just like @bar is a named variable, it has a name, it doesn't need to be autovivified, it exists by virtue of being named. By default all variables are global.

When you add my $foo; my @bar; you declare $foo and @bar to be lexical variables (not global)

it also has the side effect of satisfying use strict 'vars'; by letting it know you didn't make a typo, that you intended to have variables named $foo and @bar

neither is related to what we call autovivification ; autovivification is about using references to create complex data structures with less clicks of the keyboard

autovivification does not refer to declaring (naming) variables, it is a feature that saves you typing, it saves you from having to write  $foo[6] ||= []; $foo[6][6]=6; you can simply write  $foo[6][6] = 6; and  $foo[6] autovivifies/ becomes an arrayref

explicit, assigning an array ref to  $foo[6] ||= []; $foo[6][6]=6;

implicit, autovivifying an array ref, treat it like an array ref, it becomes an array ref  $foo[6][6] = 6;

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