You are correct so far.

@allrefs is an array. To get an element out of an array, you use a syntax like $array[ INDEX ]. To get multiple elements out of an array, you'd use @array[ INDEX1, INDEX2, ..., INDEXN ]. That's what's going on here: we sort the values of the %uni_refs hash, and then use those values as the indexes of elements to extract from @allrefs and store them in @refs.