# Pastebin idzFo8NL
class ImmutableDS {
   has $!last-return;
   has $.field-value;
   has $.some-field is built(:bind) is rw = Proxy.new:
      FETCH => -> $ { my $tmp = $!last-return; $!last-return = Nil; $tmp // $!field-value },
      STORE => -> $, $field-value { $!last-return = self.new: :$field-value }
   ;
}


my ImmutableDS $immutable-data-structure .= new: :42field-value;
my $new-immutable-data-structure = $immutable-data-structure.some-field = 13;
say $new-immutable-data-structure.some-field;
say $immutable-data-structure.some-field;


$immutable-data-structure.some-field = 3.14;
say $new-immutable-data-structure.some-field;
say $immutable-data-structure.some-field;