sub write_file {
    my ($cinfo, $data, $fields) = @_;
    . . .
    if (!defined $fields) {
        # caller didn't specify field order, so we need a list
        # of data items, so get 'em from the hash
        $fields = [ keys %$data ];
    }
    for my $field_name (@$fields) {
        my $value = $$ref{$field_name};
        print $FH "$field_name, $value\n";
    }
    . . .
}

. . .
write_file($config{'info'}, $ref, [qw(bandwidth distance size active)] );