# Create ARRAYref to list of 7 elements, with first element "",
    # remaining 6 elements copied from @fields.
    $file2{$id}=["",@fields];
    $file3{$id}=["",@fields];
...
    # trying to access 6th element of $file2{$id} as a list, but it's
    # an ARRAYref to a list with 7 elements...
    my @fields2=$file2{$id}[5];
    my @fields3=$file3{$id}[5];