package Package1;
use fields qw(attr1 attr2 attr3);

sub new {
    my Package1 $self = shift;
    $self = fields::new($self) unless ref $self;
    $self->{attr1} = "init_1";
    $self->{attr2} = "init_2";
    $self->{attr3} = "init_3";
    return $self;
}

1;

##</code><code>##

use Package1;
use Data::Dumper;

my Package1 $p = Package1->new;
$p->{attr1} = "set_1";
print $p->{attr1}, "\n";
$p->{attr4} = "set_4"; #this should fail, and did fail

##</code><code>##

use Package1;
use Data::Dumper;

my Package1 $p = Package1->new;
$p->[0]->{attr4} = 4;
$p->[$p->[0]->{attr4}] = "set_4";
print $p->[0]->{attr4};#the above two steps sunccessfully modified the internal pseudo hash
print Dumper($p);#again proves that I successfully modified the internal pseudo hash
#print $p->{attr4};#this fails