If you haven't heard, local::lib is an awesome module that makes it a snap for unprivileged users to set up a local Perl module library. One way it does that it by making it easy to set PERL5LIB and the default installation path for ExtUtils::MakeMaker and Module::Build in your .bashrc or equivalent.
For ExtUtils::MakeMaker, it just sets an environment variable, PERL_MM_OPT. For Module::Build, it currently creates a options file, .modulebuildrc, inside the local perl library and uses an environment variable to make that the default options file.
That's a bit annoying, because it puts the options file in an unexpected place and does so a bit too magically. A user of local::lib might never realize it's there, create their own ~/.modulebuildrc and wonder why it's not working.
So, thanks to a special request from mst, the next release of Module::Build will include support for a new environement variable, PERL_MB_OPT, to make local::lib's job that much easier and with fewer surprises for end users.