Note:   Just to be absolutely clear about what this is ... it is a hash of array references.   (“Arrayrefs.”)

The thing that is in a particular slot, of an array or of a hash or of a list, is always “a single value,” but one of the possible “values” that it can be, is “a reference to” absolutely anything else.   This is the magic of Perl data structuring, and one of the hallmark features of the language.   It allows arbitrary (and efficient!) data structures to be built out of a handful of simple primitives.