I know it's confusing but => is not a hash notation (ie not exclusively) like : in JS or python.

it's just a list separator, see "fat comma". I.e. you are passing a list.

There are two ways to pass hashes , either as a flat list and you have to copy @_ later to a %arg hash. (See example link)

or as a anonymous hash in curlies and you have to read the hash ref from $_[n] (see my other post)

HTH ! =)