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is rw
Combined from primary sources listed below.

In Routine§

See primary documentation in context for trait is rw 

multi trait_mod:<is>(Routine $r, :$rw!)

When a routine is modified with this trait, its return value will be writable. This is useful when returning variables or writable elements of hashes or arrays, for example:

sub walk(\thing, *@keys) is rw {
    my $current := thing;
    for @keys -> $k {
        if $k ~~ Int {
            $current := $current[$k];
        }
        else {
            $current := $current{$k};
        }
    }
    $current;
}

my %hash;
walk(%hash, 'some', 'key', 1, 2) = 'autovivified';

say %hash.raku;

produces

("some" => {"key" => [Any, [Any, Any, "autovivified"]]}).hash

Note that return marks return values as read only; if you need an early exit from an is rw routine, you have to use return-rw instead.

In Type system§

See primary documentation in context for trait is rw 

sub trait_mod:<is>(Mu:U $type, :$rw!)

The trait is rw on a class will create writable accessor methods on all public attributes of that class.

class C is rw {
    has $.a;
};
my $c = C.new.a = 42;
say $c; # OUTPUT: «42␤»