This is a well written article on the wonders of overloading and OO. But I have one complaint:

unless( ref($b) =~ /\S/ ){}
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Is not a good way to find out if a variable is a reference to a Complex or not. The better way is UNIVERSAL::isa(). Every OO class inherits from UNIVERSAL, and so you can always call $object->isa(), like this:

unless( $b->isa( 'Complex' ) ){}
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Other then that nit-pick, this is a good tutorial.