use strict; use warnings;

my %lights = map { $_ => 1 } ( 1..20_000 );

foreach my $flipper ( 2..20_000) {
    for( my $i = $flipper; $i <= 20_000; $i += $flipper ){
        $lights{ $i } = !$lights{ $i };
    }
}

# print 'em
print join(', ', grep { defined $_ }
                 map { $lights{ $_ } ? $_ : undef } 
                     ( 1..20_000)
      )."\n";