And nowadays, you can have the magic of dualvars for your own evil plots applications, using dualvar() from Scalar::Util.

BTW another typical magical dualvar value is $!, which returns an error message as a string, and an error number as a number. For example: 

local $\ = "\n"; 
$! = 3;       # error number
print $!;     # error message
print 0+$!;   # error number

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In reply to Re^2: Logical expression evaluation (dualvar) by bart
in thread Logical expression evaluation by nghosh

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