'constant::defer' creates a subroutine which on the first call runs given
code to calculate its value, and on the second and subsequent calls just
returns that value, like a constant. The value code is discarded once run,
allowing it to be garbage collected.

Deferring a calculation is good if it might take a lot of work or produce a
big result, but is only needed sometimes or only well into a program run.
If it's never needed then the value code never runs.

A deferred constant is generally not inlined or folded (see the
perlop/Constant Folding manpage) like a plain 'constant' since it's not a
single scalar value. In the current implementation a deferred constant
becomes a plain one after the first use, so may inline etc in code compiled
after that (see the /IMPLEMENTATION manpage below).