I find C++ much more productive that Perl - I guess it's just a question of what you know <g>.

Well, I know (or perhaps, knew once) C++ also, and I find Perl infinitely more productive. But then again, I hated C++ from my first encounter. Time and experience only made things worse -- but I know I am in a minority in that view:)

BTW, if I:

my $spamtest = SpamAssassin::Mail::SpamAssassin->new(...) : shared;
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will $spamtest be available in the clone if I perl_clone after this Perl code is executed?

I've never seen anyone use that syntax before, and a quick test seems to indicate that you cannot use :share in that way.

use threads; 
use threads::shared; 
use Benchmark::Timer;

$T = Benchmark::Timer->new() : shared;

Unquoted string "shared" may clash with future reserved word at (eval 
+3) line 1, <> line 2.
syntax error at (eval 3) line 1, near ") :"
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Any attempt to share a blessed reference usually results in an error.

use threads; 
use threads::shared; 
use Benchmark::Timer;
{ 
    my $T : shared; 
    $T = Benchmark::Timer->new();
}
Invalid value for shared scalar at (eval 5) line 1, <> line 5.
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So, I think the short answer is no.

I assume you were hoping to share a single instance of spamassasin between the multiple interpreters. This will not work.

Creating a unique instance in each interpreter would(may) probably work, but I have no idea how much memory that would consume.

That would require a piece of suck-and-see engineering I am afraid. Unless anyone else knows better.