BEGIN {
    if(  ! eval { require Module::X; 1 }  ) {
        die "You can't use feature X because Module::X is not installed.\n"
          if  need_module_X();
        sub A { croak "Not available" };
        sub B(\%);  *B= \&A;
        sub C();  *C= \&A;
    } else {
        Module::X->import(  qw( A B C )  );
    }
}