explain what is going on with the arrow in this context, when nothing is being dereferenced

As already explained above, you are in fact dereferencing $_ which in the context of your map is a reference to a hash.

the arrow operator, which I though was just for class methods and deferencing

Here's a summary to avoid total confusion:

There are 3 cases where the array dereferences:

- $arrayref->[0];
- $hashref->{"key"};
- $coderef->("argument");

Then there are two cases for method calls:

- $object->method; # passes $object as first argument
- SomePackage->method; # passes "SomePackage" as first argument

hth