Hash elements are created when accessed in lvalue context. Here are examples of hash values used in lvalue context: 

$hash{$key} = ...;

++$hash{$key};

for ($hash{$key}) { ... }

\$hash{$key}

some_sub($hash{$key})   # Exception!! See footnote.

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// doesn't create an lvalue context for its operands, not even if it's evaluated in an lvalue context itself. 

$ perl -e'my %h;  1 for       $h{k};  CORE::say 0+keys(%h)'
1

$ perl -e'my %h;  1 for $x // $h{k};  CORE::say 0+keys(%h)'
0

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  1. When passing a hash value to a sub, Perl specifically avoids creating the hash element until necessary. This means that some_sub($hash{$key}) doesn't create $hash{$key} unless the sub does something like $_[0] = "abc";.

In reply to Re: Are defined or // garantee not to autovivify? by ikegami
in thread Are defined or // garantee not to autovivify? by NisSam

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