This is called coercion or unmarshalling. Using Moo + Types::Standard makes it pretty easy...

use v5.12;
use strict;
use warnings;

package ToJson {
    use Moo::Role;
    
    sub TO_JSON {
        my $self = shift;
        return { %$self };
    }
}

package Named {
    use Moo::Role;
    use Types::Standard -types;
    
    has NAME => (
        is       => 'bare',
        isa      => Str,
        reader   => 'getName',
    );
}

package MyClassA {
    use Moo;
    use Types::Standard -types;
    
    with 'ToJson', 'Named';    
    
    has MY_CLASS_B => (
        is       => 'bare',
        reader   => 'getClassB',
        writer   => 'setClassB',
        isa      => InstanceOf->of('MyClassB')->plus_constructors( Has
+hRef, 'new' ),
        coerce   => 1,
    );
}

package MyClassB {
    use Moo;
    
    with 'ToJson', 'Named';    
}

use JSON::MaybeXS;

my $a = MyClassA->new(NAME => 'A instance');
my $b = MyClassB->new(NAME => 'B instance');
$a->setClassB($b);

my $json = JSON->new->convert_blessed->encode($a);
say $json;

my $inflated = MyClassA->new( JSON->new->decode($json) );
say $inflated->getClassB->getName;
[download]

The key part is this:

        isa      => InstanceOf->of('MyClassB')->plus_constructors( Has
+hRef, 'new' ),
        coerce   => 1,
[download]

This means that when the MY_CLASS_B attribute gets passed a hashref instead of an instance of MyClassB, it should call MyClassB->new on that hashref. This coercion happens in both the constructor (new) and the attribute writer (setClassB).

Of course, it can be done without Moo or Types::Standard. They're not magic. To do it manually, alter your MyClassA::new to check %extras to find if there's any values that need inflating. Something like this:

    if ( exists $extras{MY_CLASS_B} and ref $extras{MY_CLASS_B} eq 'HA
+SH' ) {
        $extras{MY_CLASS_B} = MyClassB->new( %{ $extras{MY_CLASS_B} } 
+);
    }
[download]

And probably a good idea to do something similar in setClassB. Allow it to accept a hashref and inflate it to a MyClassB object.

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