Facility for creating read-only scalars, arrays, hashes

Readonly.pm provides a facility for creating non-modifiable scalars,
arrays, and hashes.

Readonly.pm
* Creates scalars, arrays (not lists), and hashes.
* Creates variables that look and work like native perl variables.
* Creates global or lexical variables.
* Works at runtime or compile time.
* Works with deep or shallow data structures.
* Prevents reassignment of Readonly variables.