This is just for my or for future readers' reference... Please correct me if there's anything wrong or not explained properly. One reason I type this out is because I see code examples that use voodoo I'm not yet accustomed to. So maybe I can help someone else that reads this in the future. =)

The expression $_ & 1 will evaluate to zero (false) for any even number $_. This uses the bitwise operator &...

Translate both numbers ($_ and 1) to their binary formats and fill the lesser number with leading zeros (13 => 1101, 1 => 0001)... Perform the AND comparison between the numbers vertically and return that result. Since 1 is a number with many leading zeros, it cancels/falses out all higher order bits greater than 1 for the first number. The only way to make this expression true is to compare the two first bits as 1 AND 1. 1 is an active bit in binary when the first number is odd.