I don't suppose you'd care to insert some parentheses,

I don't think adding unnecessary parens would help at all :)

or explain it?

Sure:

@{ $netifaces->{ $_ } }{ @nets } = (1) x @nets for @eths;
   ^^^^^^^^^^ a hash reference

@{ $netifaces->{ $_ } }{ @nets } = (1) x @nets for @eths;
   ^^^^^^^^^^^^^^^^^^ an element in that hash

@{ $netifaces->{ $_ } }{ @nets } = (1) x @nets for @eths;
                                   iterated over ^^^                  
+                            

@{ $netifaces->{ $_ } }{ @nets } = (1) x @nets for @eths;
                                      by these keys ^^^^^

@{ ... }{ @nets }
^^^^^^^^^^^^^^^^^^ a hash slice with the keys in @nets

@{ ... }{ @nets } = (1) x @nets ...;
       is assigned ^ ^^^^^^^^^^^ a list of '1's the size of @nets
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Putting that all together, we get:

For each value in @eths, create a key in $netifaces with a value that consists of a hash containing all of the keys in @nets, each with the value 1.

Eg.

@{ $netifaces->{ $_ } }{ @nets } = (1) x @nets for @eths;
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