JSON::XS refuses to encode a restricted hash to json (while Cpanel::JSON::XS is not).

json_of_restricted_hash.pl

#!/usr/bin/env perl
use strict;
use warnings;

{

  my ($use_cpanel) = @ARGV;

  my $class = do {
    if ($use_cpanel) {
      require Cpanel::JSON::XS;
      'Cpanel::JSON::XS'
    } else {
      require JSON::XS;
      'JSON::XS';
    }
  };

  my $m = Mail->new(from => 'foo', to => 'bar', subject => 'xxx', message => 'yyy');

  print $class->new->convert_blessed->encode($m), "\n";
}

#========================================

package Mail {
  use fields qw/from to subject message/;
  sub new {
    my $pack = shift;
    my $self = fields::new($pack);
    %$self = @_;
    $self;
  }

  sub TO_JSON {
    my $self = shift;
    +{%$self};
  }
};

result.txt

Z-chtholly(pts/0)% ./restricted_hash_json.pl
Modification of a read-only value attempted at ./restricted_hash_json.pl line 21.
Z-chtholly(pts/0)% ./restricted_hash_json.pl 1
{"subject":"xxx","from":"foo","message":"yyy","to":"bar"}