>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;
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Update: Why are you checking for defined? define accepts a scalar, not specifically a boolean. False is defined, so your tests should all print 1 if you were truly examining lists in boolean contexts. (False is 0 in numerical context and "" in string context.) To check the value of a list in a boolean context, simply run the following:

>perl -wle "print(()?'True':'False');"
False
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I think your question is invalid. Just like there's no such thing as a list in scalar context, I'm guessing that there is no such thing as a list in a boolean context. It's up to the functions returning lists to provide the value.