Paraphrasing from from perlop:The left side of the "->" ( infix dereference operator) must be a REFERENCE to ....

In your case, you are supplying a HASH (Not a reference).

Just get rid of the %{...} , and you will have a (Hash) reference.

             return $WAI->kBToMB( $WAI->getMem()->{MemTotal} );
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(No need for quotes inside {} ).