The simplest expression of this form is probably

$hash{foo} ||= exists $hash{foo};

versus

$hash{foo} = $hash{foo} || exists $hash{foo};

In reply to Re: Are "$hash{$_} ||= 1 + keys %hash" and variants well defined or not? by whio
in thread Are "$hash{$_} ||= 1 + keys %hash" and variants well defined or not? by demerphq

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