my %log_handler;

sub log_handler
{
  my ($self,$sub)=@_;
  return 0 unless ref $sub eq 'CODE';
  $log_handler{$self}=$sub;
  return 1;
}

#...

sub new
{
  #...

  $log_handler{$self}=sub{ 1 };
  return $self;
}

#...

  $log_handler{$self}->('type' => \@messages);

#...