print $a+$ref_b; #coerce reference into number 

Those are both scalars. Not a type conversion.
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Humm, no! To make any arithmetic operation we need to convert the string (that are just a sequence of bytes), into numbers, where we use 4 bytes (32 bits processor) to make the calculation. (this is standart calculation ok!)

The wonderful job that Perl does for us, with speed, is to convert automatically all the boolean, integer, floating and string types with a single type, SCALAR.