>perl -MO=Deparse -wle "print defined +() ? 1 : 0"
print defined(()) ? 1 : 0;

>perl -MO=Deparse -wle "$_ = 1; print defined +() ? 1 : 0"
$_ = 1;
print defined(()) ? 1 : 0;

>perl -MO=Deparse -wle "$_ = 1; print defined () ? 1 : 0"
$_ = 1;
print defined $_ ? 1 : 0;