From language/operators

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Binding operator. Whereas $x = $y puts the value in $y into $x, $x := $y makes $x and $y the same thing.

my $a = 42;
my $b = $a;
$b++;
say $a;

This will output 42, because $a and $b both contained the number 42, but the containers were different.

my $a = 42;
my $b := $a;
$b++;
say $a;

This will output 43, since $b and $a both represented the same object.

If type constrains on variables or containers are present a type check will be performed at runtime. On failure X::TypeCheck::BindingType will be thrown.

Please note that := is a compile time operator. As such it can not be referred to at runtime and thus can't be used as an argument to metaoperators.