sub list_entries {
    my ($filename) = @_;
    ...
    return { $key => { ...}, $key => {...}, .. ];
}

sub add_entry {
    my ($filename, $key, %data) = @_;
    ...

sub delete_entry {
    my ($filename, $key) = @_;
    ...

sub update_entry {
    my ($filename, $key, %data) = @_;
    ....

##</code><code>##

package MyDatabase;

sub new {
    my $class = shift;
    my $self = {};
    bless $self, $class;
    return $self;
};
  
sub set_schema {
    my ($self, @fields) = @_;
    $self->{'fields'} = [@fields];
};

sub set_filename {
    my ($self, $filename) = @_;
    $self->{'filename'} = $filename;
};

sub read_entries {
    my ($self) = @_;
    $self->{'data'} = {};
    open(F,"<".$self->{'filename'}) || die "Could not open ".$self->{'filename'}." to read";
  
    while (my $line = <F>) {
        my ($key, @field_values) = split(/:/,$line);
        for (@{$self->{'fields'}}) {
            $self->{'data'}{$key}{$_} = shift(@field_values);
        }
    }

    close(F);
};

sub write_entries {
    my ($self) = @_;
    open(F,">".$self->{'filename'}) || die "Could not open ".$self->{'filename'}." for writing";

    for my $key (keys %{$self->{'data'}}) {
        # Prints out the key then the data fields in order.
        print F join(':',($key, map {$self->{'data'}{$key}{$_}} @{$self->{'fields'}}))."\n";
    }

    close(F);  
}

sub list_entries {
    my ($self) = @_;
    $self->read_entries();
    return %{$self->{'data'}};
}

sub set_entry {
    my ($self, $key, %data) = @_;
    eval { $self->read_entries(); }; # drop exceptions for the read on set, if the db doesn't exist, we'll create it.
    $self->{'data'}{$key} = \%data;
    $self->write_entries();
}

sub delete_entry {
    my ($self, $key) = @_;
    $self->read_entries();
    delete $self->{'data'}{$key};
    $self->write_entries();
}

##</code><code>##

my $db = new MyDatabase();

$db->set_filename("test.db");
$db->set_schema("first name","last name","email");

$db->set_entry("testentry",(
    "first name" => "Phi",
    "last name" => "RatE", 
    "email" => 'this@is.a.test.com'));
 
my %entries = $db->list_entries();

for (keys %entries) {
    for my $field (keys %{$entries{$_}}) {
        print "$field -> ".$entries{$_}{$field}."\n";
    }
}