This module corrects the speed problem, at least with respect to scalar
variables. When Readonly::XS is installed, Readonly uses it to access the
internals of scalar variables. Instead of creating a scalar variable object and
tying it, Readonly simply flips the SvREADONLY bit in the scalar's FLAGS
structure.

Readonly arrays and hashes are not sped up by this, since the SvREADONLY flag
only works for scalars. Arrays and hashes always use the tie interface.

Programs that you write do not need to know whether Readonly::XS is installed or
not. They should just "use Readonly" and let Readonly worry about whether or not
it can use XS. If the Readonly::XS is present, Readonly will be faster. If not,
it won't. Either way, it will still work, and your code will not have to change.

Your program can check whether Readonly.pm is using XS or not by examining the
$Readonly::XSokay variable. It will be true if the XS module was found and is
being used. Please do not change this variable.

WWW: http://search.cpan.org/dist/Readonly-XS/