Not a variable, but a value: the value for false, which you can generate with !1.
It return '' in string context and 0 in numeric context, and doesn't warn in numeric context like the empty string normally does:
$ perl -wE 'say 0+""'
Argument "" isn't numeric in addition (+) at -e line 1.
$ perl -wE 'say 0+!1'
My version of Scalar::Util doesn't export an is_dual function, so I can't check. It helps to type that without the _ :-)
Update: I should have mentioned it earlier: even though the thing is called a "dualvar", it's not actually variables that are tested; values are being tested. It makes no difference if those values are return values, stored in array, or are stored in a scalar variable.
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