package EventRepository;
use Moose;

has 'future_existing_events' => (
  is => 'rw',
  isa => 'HashRef',
  lazy => 1,
  default => sub { {} },
  trigger => \&_get_future_events('existing_events'),
);

sub _get_future_events {
  print Dumper(\@_);
  ...
}