AST is a library for manipulating abstract syntax trees.
It embraces immutability; each AST node is inherently frozen at creation, and updating a child node requires recreating that node and its every parent, recursively.
This is a design choice. It does create some pressure on garbage collector, but completely eliminates all concurrency and aliasing problems.
See also AST::Node, AST::Processor::Mixin and AST::Sexp for additional recommendations and design patterns.