my $subclass = Module::Build->subclass(
    class   => 'My::Builder',
    code    => &_custom_code(),
);

my $build = $subclass->new(
    module_name        => 'YourName',
    script_files       => [ qw( same as above ) ],
    #...
);

$build->create_build_script;

sub _custom_code {
    return <<'EO_CODE';
    sub ACTION_install {
        # do whatever you want at the end of install
    }
EO_CODE
}