Perl Moose add instance attribute not class attribute - perl

Question

I need to add attribute to Moose class instance. In the code below, when I create instance of the class Child and add attribute "app" to it, I find this attribute also added when I create next instances. What I am doing wrong, again I need the attribute per created instance.
#!C:\perl\bin\perl.exe
#!/usr/bin/perl
use v5.10;
use Moose;
use Data::Dumper;
{
package Child;
use Moose;
use utf8;
sub name {
say "My name is Richard";
}
}
sub add_attribute {
my ($object, $attr) = #_;
my $meta = $object->meta;
if (!$object->can("app")) {
$meta->add_attribute(app => (is => 'rw', default => sub{$attr}));
$object->app($attr);
}
else {
#$object->app($attr);
say "attr $attr already exists: object=". ref($object) . ", attr=".($object->app);
}
}
my $child = Child->new;
$child->name;
add_attribute($child, "First");
say "Child Attr: " . $child->app;
say "";
say Dumper($child);
my $child1 = Child->new;
$child1->name;
#add_attribute($child1, "Second");
say "Child1 Attr: " . $child1->app;
say Dumper($child1);
#say Dumper($child1->meta);
output:
My name is Richard
Child Attr: First
$VAR1 = bless( {
'app' => 'First'
}, 'Child' );
My name is Richard
Child1 Attr: First
$VAR1 = bless( {
'app' => 'First'
}, 'Child' );

Answer 1

The trick is to create a new subclass of your original class, add the attribute to that, then rebless the instance into the new subclass. Here's an example:
use v5.14;
package Person {
use Moose;
has name => (is => 'ro');
}
sub add_attribute {
my ($obj, $name, $value) = #_;
my $new_class = Moose::Meta::Class->create_anon_class(
superclasses => [ ref($obj) ],
);
$new_class->add_attribute($name, is => 'rw');
$new_class->rebless_instance($obj, $name => $value);
}
my $alice = Person->new(name => 'Alice');
my $bob = Person->new(name => 'Bob');
add_attribute($alice, foot_size => 6);
say $alice->foot_size;
say $bob->foot_size; # dies, no such method