This module was written because I stumbled on some serious issues of Readonly
that aren't easily fixable without breaking backwards compatibility in subtle
ways. In particular Readonly's use of ties is a source of subtle bugs and bad
performance. Instead, this module uses the builtin readonly feature of perl,
making access to the variables just as fast as any normal variable without
the weird side-effects of ties. Readonly can do the same for scalars when
Readonly::XS is installed, but chooses not to do so in the most common case.