Perl 6 allows you to write it in one loop, if you prefer:

for @eths X @nets -> $eth, $net {
  %netifaces{$eth}{$net} = 1;
}
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Rakudo and Niecza both support it.

You can also do something like

for @eth {
    %netifaces{$eth} = ( @nets X=> 1 ).hash
}
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See this blog post for a description of what X and X=> do.

Update: fixed variable name in the first code example, TimToady++