It seems possible some people have 64 bit Perl builds, and only a 32-bit libstdc++

That does indeed seem possible. And if the test executable (that the Makefile.PL runs) fails to build, then the assumption is made that iostream is unavailable.
But, of course, that is not necessarily so. It could also be that the failure has occurred because there is no libstdc++ for the g++ compiler that has been chosen.

I suggest the Makefile.PL could first run this test script: 
int main(void) {
 return 0;
}

[download]
AFAICT, if that test script builds ok, then you know a suitable libstdc++ for the chosen g++ compiler has been found.
Once it is established that libstdc++ has been found, *then* you do the test to determine whether iostream is present.

As you know, just because there's a suitable libstdc++ for the chosen compiler doesn't necessarily mean that's the right g++ compiler for that build of perl.
But knowing that there's at least a libstdc++ for the compiler that was chosen might help narrow things down for you.

Still trying to find something that tells me how to run g++ in 64-bit mode on my deb box :-(

Cheers,
Rob

In reply to Re^6: [OT] Installing g++ compiler on ubuntu-12.04LTS by syphilis
in thread [OT] Installing g++ compiler on ubuntu-12.04LTS by syphilis

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