etc_files => {
        'etc/skycat.cfg' => 'etc/skycat.cfg',
        'etc/sss.cfg' => 'etc/sss.cfg',
    },
    install_path => {
        'etc' => File::Spec->catdir($Config{installprefix}, 'etc'),
    },

##</code><code>##

...
All tests successful.
Files=27, Tests=3453, 15 wallclock secs ( 0.19 usr  0.05 sys + 11.09 cusr  1.56 csys = 12.89 CPU)
Result: PASS
  GSB/Astro-Catalog-4.37.tar.gz
  ./Build test -- OK
Running Build install for GSB/Astro-Catalog-4.37.tar.gz
Building Astro-Catalog
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
ERROR: Can't create '/usr/etc'
mkdir /usr/etc: Permission denied at /usr/share/perl5/vendor_perl/ExtUtils/Install.pm line 489.

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 at /some/dir/perl/lib/perl5/Module/Build/Base.pm line 3576.
  GSB/Astro-Catalog-4.37.tar.gz
  ./Build install  -- NOT OK
Stopping: 'install' failed for 'Astro::Catalog'.

##</code><code>##

# Environment.
# Setting *prefix here only because of the "$Config{installprefix}".
PERL_MB_OPT=--install_base '/some/dir/perl' --config installprefix='/some/dir/perl' --config prefix='/some/dir/perl'

PERL_MM_OPT=INSTALL_BASE='/some/dir/perl' PREFIX='/some/dir/perl'

# In the .cpan/build directory.
grep -C 1 -r 'install|prefix' _build | grep -v orig
_build/build_params-       {
_build/build_params:         'prefix' => '/some/dir/perl'
_build/build_params-       },
--
_build/build_params-         'include_dirs' => [],
_build/build_params:         'install_base' => '/some/dir/perl',
_build/build_params:         'install_base_relpaths' => {},

##  "/usr" did not change.
_build/build_params:         'install_path' => {
_build/build_params-                             'etc' => '/usr/etc'
_build/build_params-                           },

_build/build_params:         'install_sets' => {},
_build/build_params:         'installdirs' => 'site',
_build/build_params-         'libdoc_dirs' => [
--
_build/build_params-         'perl' => '/usr/bin/perl',
--
_build/build_params-         'pollute' => undef,
_build/build_params:         'prefix' => undef,
_build/build_params:         'prefix_relpaths' => {},
_build/build_params-         'prereq_action_types' => [
--
_build/runtime_params-       'config' => {
_build/runtime_params:                     'prefix' => '/some/dir/perl'
_build/runtime_params-                   },
_build/runtime_params:       'install_base' => '/some/dir/perl',
_build/runtime_params-       'verbose' => '1'