There is no special handling needed to copy a string; just assign it to a new variable:
my $original = 'Hello.';
my $copy = $original;
say $copy; # prints "Hello."
$copy = 'Goodbye.';
say $copy; # prints "Goodbye."
say $original; # prints "Hello."
You can also bind a new variable to an existing one so that each refers to, and can modify the same string.
my $original = 'Hello.';
my $bound := $original;
say $bound; # prints "Hello."
$bound = 'Goodbye.';
say $bound; # prints "Goodbye."
say $original; # prints "Goodbye."