In Perl a string is mutable.

Actually in Perl 5 that's an implementation detail. You can't modify string literals, you can only mutate variables which hold strings. A (non-XS) Perl programmer wouldn't see the difference to an immutable string.

But yet, many modern languages tend towards immutable basic data types because it makes multi threading easier - an immutable value can be shared between threads without the need for locking or copying.