Can it be improved?

Possibly. Possibly not. But that one function does quite nicely demonstrate why perl internals are complex and hard to work with. That "clearly defined" op has to cope with (from a quick perusal of the src):

• both byte- or utf8-encoded strings - and having to convert between byte and char offsets - with a caching scheme for sometimes O(1) performance on long utf8 strings;
• delayed lvalue assignment, with sometimes the lvalueness only known at runtime depending on how a :lvalue sub has been called;
• being called in void, scalar or list context;
• handling a variable number of args;
• integer-valued args being either signed or unsigned (both are supported);
• get and set magic;
• being passed a reference rather than a string;
• issuing appropriate warnings;
• tainting;
• locales;

Putting all that together makes it really easy to break things, even with an extensive test suite.

Dave.