It's hard to beat nested foreach loops for simplicity and readability in the example you provided. But if there is a Perl5 alternative, it's probably found in tye's Algorithm::Loops. In specific, you could accomplish the same like this:

NestedLoops( 
    [ \@eths, \@nets ], 
    sub{ 
        $netfaces->{ $_[0] }{ $_[1] } = 1; 
    } 
);
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Your posted example is what everyone's going to expect to see. But I would guess that any reasonably experienced programmer would be able to look at the snippet posted in this node and know pretty quickly what is going on.