The Perl 5 module Readonly::XS is a companion module to the Readonly
module.  You do not use Readonly::XS directly.  Instead, once it
is installed, the Readonly module will detect this and will use it
for creating read-only scalars.  This results in a significant
speed improvement.  This does not speed up read-only arrays or
hashes.

Homepage:
https://metacpan.org/release/Readonly-XS
