my $list_item = {
     obj      => $server,
     selected => 0,
};

##</code><code>##

@servers = map  { $_->{obj} }
           grep { $_->{selected} } @servers;

##</code><code>##

     1  package Trait::Selected;
     2
     3  use strict;
     4  use warnings;
     5
     6  use Class::Trait 'base';
     7
     8  our @REQUIRES = qw(id);
     9
    10  my $get_id = sub {
    11      my $self = shift;
    12      return sprintf "%s %d" => ref $self, $self->id;
    13  };
    14
    15  my %is_selected;
    16
    17  sub is_selected {
    18      my $self = shift;
    19      my $id   = $self->$get_id;
    20      $is_selected{$id} ||= 0;
    21      return $is_selected{$id} unless @_;
    22      $is_selected{$id} = shift;
    23      return $self;
    24  }
    25
    26  1;

##</code><code>##

# This used to be $item->{selected} = 1; and is really easy to mistype
$object->is_selected(1);  # Mistyping the method becomes a runtime failure

##</code><code>##

@servers = map  { $_->{obj} }
  grep { $_->{selected} } @servers;