Couldn't the original Perl script do everything in the first place (i.e. perl Build.PL, perl Build.PL test, and so on).
in reply to Re: RFC: How to Release Modules on CPAN in 2011
in thread RFC: How to Release Modules on CPAN in 2011
You should ask on Module::Build dev list... but there is no good answer.
Originally it was for the same reason you generate Makefiles, so you can copy/rm... files you used to generate the Makefile, without them being locked/in-use.... but at some point Module::Build devs forgot that.