A binary // operator is the defaulting operator.
That is:

$a // $b
[download]

is short for:

defined($a) ?? $a :: $b
[download]

except that the left side is evaluated only once. It will work on arrays and hashes as well as scalars. It also has a corresponding assignment operator, which only does the assignment if the left side is undefined:

$pi //= 3;
[download]