my %big_hash;
    my $field_name1 = "field_name1";
    my $field_name2 = "field_name2";
    ...
    $big_hash{$key}{$field_name1}
    $big_hash{$key}{$field_name2}
    ...

##</code><code>##

    my %field_name1;
    my %field_name2;
    ...
    $field_name1{$key}
    $field_name2{$key}