use strict;
use warnings;
$|=1;

sub sol1 {
	#init;
	my $lights = 20_000;
	my @lit = map {1} (1..$lights);
	
	my @sol;
	
	#flip;
	for (2..$lights) { 
		my $cnt = $_-1;
		while ($cnt < $lights) { 
			$lit[$cnt] = !$lit[$cnt];
			$cnt+=$_;
		}
	}
	
	#answer;
	for (0..$#lit) { push @sol, $_+1 if $lit[$_] }
}

sub sol2 {
	my $lights = 20_000;
	
	my @sol;
	
	for (1..$lights) {
		my $sqr = sqrt($_);
		push @sol, $_ if int($sqr) == $sqr;
	}
}

use Benchmark ':all';
cmpthese( 100, {
	sol1 => \&sol1,
	sol2 => \&sol2,
});

__END__
       Rate  sol1  sol2
sol1 6.58/s    --  -93%
sol2 98.5/s 1398%    --