BEGIN块中的Perl模块版本信息

Question

In some CPAN modules I found the next construction

BEGIN {
    $Package::Name::VERSION = "N.N";
}

What is a rationale behind putting the package version information into the BEGIN block?

for example: http://cpansearch.perl.org/src/JSWARTZ/Poet-0.12/lib/Poet/Cache.pm

EDIT - in the most recent module version it is pulled out, but anyway - it was here - so wondering why it is good (or bad) :)

Answer 1

It's one of those things that everyone has always done and can't remember why!

Essentially it is because use statements allow multiple modules to be in compilation at the same time, so although it looks like a module is complete it may have only just started compiling.

Any code being compiled can check the value of $Module::VERSION by using the inherited UNIVERSAL::VERSION subroutine, which is already defined implicitly before a module even starts compiling.

Remember that, if Module.pm contains

use Another::Module;

Then compilation of Module.pm is put on hold while Another/Module.pm is compiled.

There is nothing to stop Another::Module from doing

use Module 1.5;

which will call Module::VERSION(1.5) (inherited from UNIVERSAL::VERSION ) to check that $Module::VERSION is 1.5 or greater.

If $Module::VERSION is defined outside a BEGIN block, it won't be set until after all use statements have been completed, and so too late for other modules to do version checking on Module .

I hope this is clear. I can't help thinking there must be a simpler explanation, but nothing comes to mind.