The latest Scalar::Util::isdual considers FLAGS = (GMG,SMG,pNOK,pPOK) to be a dualvar. (No idea about earlier versions, but I suspect it's the same.) The docs are wrong. The following returns 1 for both 5.8.9 and 5.18.1, while the docs say it returns false: 

perl -MScalar::Util=isdual -MDevel::Peek \
   -e'$!=1; print("$!\n"); print(isdual($!)?1:0,"\n"); Dump($!);'

[download]

Nit: $! is magical (like tied variables), but it's not a tied variable like the docs say.

In reply to Re^2: Dualvars besides $! by ikegami
in thread Dualvars besides $! by szabgab

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