The slash slash operator in perl returns the left side value if it is defined, otherwise it returns the right side value.
This differs from the || operator in that the || operator checks for truth rather than definendness.
$x //= 'value' assigns 'value' to $x unless $x is defined.
The slash slash operator comes in handy, for example, when handling null values with one of the DBD modules.
use warnings;
use strict;
use feature 'say';
my $p;
my $q = "foo";
my $r = "";
my $t = "0";
say $p // "bar"; # bar
say $q // "bar"; # foo
say $r // "bar"; #
say $t // "bar"; # 0
say "----------";
$p //= "V";
$q //= "V";
$r //= "V";
$t //= "V";
say $p; # v
say $q; # foo
say $r; #
say $t; # 0