sub buk1 { my $lights = '1' x ( 20_000 ); for my $gap ( 2 .. 20_000 ) { for( my $o = $gap; $o <= 20_000; $o += $gap ) { substr($lights, $o, 1) = substr($lights, $o, 1) eq '0' ? '1' : '0'; } } $answers{ buk1 } = join ', ', grep{ substr( $lights, $_, 1 ) } 1 .. 20_000; } sub buk2 { my $lights = "\xFF" x ( 20_001 / 8 ); for my $gap ( 2 .. 20_000 ) { for( my $o = $gap; $o < 20_000; $o += $gap ) { vec( $lights, $o, 1 ) = ~vec( $lights, $o, 1 ); } } $answers{ buk2 } = join ', ', grep{ vec( $lights, $_, 1 ) } 1 .. 20_000; } P:\test>junk Rate duckyd2 duckyd tanktalus buk buk2 duckyd2 1.96/s -- -1% -53% -66% -69% duckyd 1.99/s 1% -- -53% -66% -68% tanktalus 4.20/s 114% 111% -- -28% -33% buk 5.82/s 197% 193% 39% -- -7% buk2 6.27/s 220% 215% 49% 8% -- Comparing buk1 and buk2 Comparing buk2 and duckyd Comparing duckyd and duckyd2 Comparing duckyd2 and tanktalus