Facility for creating read-only scalars, arrays, hashes

Readonly provides a facility for creating non-modifiable scalars, arrays, and hashes. Any attempt to modify a Readonly variable throws an exception. Readonly: * Creates scalars, arrays (not lists), and hashes * Creates variables that look and work like native perl variables * Creates global or lexical variables * Works at run-time or compile-time * Works with deep or shallow data structures * Prevents reassignment of Readonly variables