immutable.pl

use strict;
use warnings;
use v5.20;


package ImmutableString {
  use Hash::Util qw/lock_hash/;

  sub new {
    my ($cls, $val) = @_;

    my $self = {
      val => $val,
    };

    bless $self, $cls;
    lock_hash(%$self);
  }

  sub get { shift->{val} }

}


my $s = ImmutableString->new('hello');
# fail
$s->{val} = 'foo';

# fail
substr($s->{val}, 0, 1, 'H');

# print "hello"
say $s->get;