use feature 'signatures';

my $on = 0;

sub switch_lamps ($switch_to) {
  state $lamp_state = 0;
  return if $lamp_state == $switch_to;
  $lamp_state = $switch_to;
  # ...
}

sub is_nighttime {
  #...
}

while (1) {
  switch_lamps( $on && is_nighttime() );
  sleep(60);
}