_attribute_default returns the same references every time, as the default attributes of new objects. Every $self->{_hash} refers to the same hash by default. You reassign the _data element by providing an initial value, but you don't reassign the _hash element.

Watch it happen by adding some print statements to your constructor:

# Constructor
sub new {
    my ( $class, %arg ) = @_;
    my $self = bless {}, $class;

    $self->_arg_exist(%arg);    # Check if all given args are ok

    # Set the attributes for the provided arguments
    foreach my $attribute ( $self->_all_attributes() ) {
        my ($argument) = ( $attribute =~ /^_(.*)/ );
        # Initilize to defaults
        $self->{$attribute} =
          $self->_attribute_default($attribute);
        # Override defaults with arguments
        if ( exists $arg{$argument} ) {
print STDERR "Overriding $attribute with $argument ($arg{$argument})\n
+";
            $self->{$attribute} = $arg{$argument};
        }
    }
foreach my $k (keys %$self) {
  print STDERR "$k: $self->{$k}\n";
}


     # Empty the hash loci
    %{$self->{_hash}} = ();

    # Getting the data from the array
    # and storing the information in the hash
    $self->_get_hash ();

    return $self;
}
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