Depending on when you want to set $self->{_not_exposed_reason}, 
sub not_exposed_reason {
    my ($self, $not_exposed_reason) = @_;
    return
        $self->{_not_exposed_reason} = defined($not_exposed_reason)
            ? $not_exposed_reason
            : "No exposed reason";
}

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or 
sub not_exposed_reason {
    my ($self, $not_exposed_reason) = @_;
    if (defined($not_exposed_reason)) {
        return $self->{_not_exposed_reason} = $not_exposed_reason;
    } else {        
        return "No exposed reason";
    }
}

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Note that 

defined(X) ? X : Y

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can be written as 
X // Y

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since Perl 5.10.

In reply to Re: Question about defined loop by ikegami
in thread Question about defined loop by siddheshsawant

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