@{ $n{ $mag } }{ @keys } = ( 0 ) x @keys;

##</code><code>##

( 0 ) x @keys # A list of zeroes the same length as @keys
=             # is assigned to
{ @keys }     # a slice using @keys for the keys
@{            # in the hash referred to
  $n{$mag}    # by the value pointed to by $mag in %n.
}