while (my ($name, $hash) = each %nfs) {
    for my $devloc (ref $hash->{'%filer_device'} ? keys %{$hash->{'%fi
+ler_device'}} : '%filer_device') { # hack here
        my $device = (ref $hash->{'%filer_device'} ? $hash->{'%filer_d
+evice'} : $hash)->{$devloc}; # and here
        my $volume = ref $hash->{'%filer_volume'} ? $hash->{'%filer_vo
+lume'}{$devloc} : $hash->{'%filer_volume'};
        print "\@dev $name $device $volume $hash->{'%export_name'}\n";
    }
}
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This code uses a small hack providing itself a fake hash and hash key when '%filer_device' value is not a hash reference. The downside of it is lack of data structure checking, so the real solution should be more verbose (and probably use Params::Validate, Data::Validator, Data::Diver or Data::DPath).