sub makeMixin {
  #input is an array of file names
  my @aImpliedPackageNames = map { ... } @_;

  my $class;
  eval {
    $class = MyClass->class(@aImpliedPackageNames);
    return 1;
  } or do {
     my $e=$@;
     # compute exception of your choosing, e.g.
     # a list of missing modules:

     my @aNotFound;
     foreach my $sPackage (@aImpliedPackageNames) {
       if (!eval("scalar keys %${sPackage}::")) {
          push @aNotFound, $sPackage;
       }
     }

     if (scalar(@aNotFound)) {
        die "Couldn't make mixin: the following packages "
            . "are undefined: @aNotFound";
     } else {
        die $e;
     }
  }
  return $class;
}