I tried to find an elegant way to avoid map...grep

Roger's first solution actually does this in a pretty nice way. I added some formatting to [hopefully] make it more clear, but here's his code:

    my %hashslice1 = map { /^NETWORK/
                           ? ($_, $hash{$_})
                           : (             )
                         } keys %hash;
[download]

Even still, I can see how mixing the selection of the keys with the hashification is a bit confusing, so that's why I gave a map {} grep {} solution that separates them.