I think you are misinterpreting the results of your test. (No, I was misunderstanding what point was being made. ;)

Scalar::Util::isdual returns a true value if the variable being tested is a dualvar. Your test indicates that isdual is returning 1 when tested against $!, which is a true value, indicating that $! is a dualvar.

Update: A list of "dualvar" special variables, as I find them.

• $( The documentation eludes to this; it may contain a space-delimited list (a string), but you can only assign a number to it.
• $) The documentation is similar on this one, and it too is a dualvar.
• $^E The POD says "Caveats mentioned in the description of $! generally apply to $^E , also."

Also, since $_ implicitly topicalizes a foreach loop, and since it's an alias to an iteration's topic, it can be made to appear to be a dualvar:

for ( $! ) {
  print Scalar::Util::isdual($_), "\n";
}
[download]

...outputs '1', because in this case $_ is an alias to a dualvar. So we can add to the list:

• Any variable that aliases a dualvar becomes a dualvar while the alias is in effect (consider foreach ( $! ) { # $_ is a dualvar here })