'constant::defer' creates a subroutine which on the first call runs given
code to calculate its value, and on any 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) since it's not a single scalar value. In
the current implementation a deferred constant becomes a plain constant
after the first use, so may inline etc in code compiled after that (see the
/IMPLEMENTATION manpage below).
See examples/simple.pl in the constant-defer source code for a complete