The Readonly module is an effective way to create non-modifiable variables. However, it's relatively slow.

The reason it's slow is that is implements the read-only-ness of variables via tied objects. This mechanism is inherently slow. Perl simply has to do a lot of work under the hood to make tied variables work.

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.