a

And one final time – precedence allows us to do away with the `(`/`)`s – which comes down to just ***14*** bytes of “perl”y goodness.

`pop;` – put the last (only) (first) parameter into `$_`
`/./` – matches the first character of `$_` and stores the result in `$&` and more importantly stores the match to the right of this in `$'`. The regex operator `//` operates on `$_` by default if no other variable specified
`$'` – which is the numeric part of the square
`ord` – get character code of first character of `$_` – like the regex operator this works on `$_` by default.
`+` – add `$'` {the number part} to `ord` {the numeric code of the letter} as `+` has precedence over `&` to get a number for each square which is ODD when light and EVEN when dark, this is done first so we DON’T need braces `(` & `)`.
`&` – bitwise and with `/\d/` which will always be `1` with the result of `$'+ord` which results in getting the last (unit) bit of `$'+ord` which is what we want.

Which gives us….