It is failing to test correctly and I've traced the problem to the following.

sub AUTOLOAD {
    # This AUTOLOAD is used to 'autoload' constants from the constant(
+)
    # XS function.  If a constant is not found then control is passed
    # to the AUTOLOAD in AutoLoader.

    my $constname;
    ($constname = $AUTOLOAD) =~ s/.*:://;
    my $val = constant($constname);
    if ($! != 0) {
    if ($! =~ /((Invalid)|(not valid))/i || $!{EINVAL}) {
        $AutoLoader::AUTOLOAD = $AUTOLOAD;
        goto &AutoLoader::AUTOLOAD;
    }
    else {
      croak "Your vendor has not defined SSLeay macro $constname";
    }
    }
    eval "sub $AUTOLOAD { $val }";
    goto &$AUTOLOAD;
}
[download]

static double
constant(char* name)
{
    errno = 0;
    switch (*name) {
    case 'A':
    if (strEQ(name, "AT_MD5_WITH_RSA_ENCRYPTION"))
#ifdef SSL_AT_MD5_WITH_RSA_ENCRYPTION
        return SSL_AT_MD5_WITH_RSA_ENCRYPTION;
#else
        goto not_there;
#endif
    break;
      /* Snip similar for B..Z */
    case '_':
    if (strEQ(name, "_TEST_INVALID_CONSTANT"))
        goto not_there;
    }
    errno = EINVAL;
    return 0;

not_there:
    errno = ENOENT;
    return 0;
}
[download]

The problem is that setting errno within the XS code is not propagated to the Perl $! variable.

The build process does keep warning me...

*** Be sure to use the same compiler and options to compile your OpenSSL, perl, and Net::SSLeay. Mixing and matching compilers is not supported.

I guess I shouldn't have ignored that warning. Or rather I shouldn't have thought it meant I should mix GCC v Microsoft v Intel compilers.

My Perl build is a binary from ActiveState, v5.8.8 MSWin32-x86-multi-thread build 820. This was built using the Microsoft compiler. My compiler is Microsoft 32-bit C/C++ Optimizing Compiler Version 13.10.3077 for 80x86. I'm guessing these are sufficiently "different" that the errno symbol ends up bound to different variables in auto/Net/SSLeay.dll from it is in perl58.dll. My OpenSSL is a binary download from somewhere.

I've noticed that a lot of people (not just Perl users) seem to use Windows, and many of them download programs/libraries as compiled binaries rather than source. (Indeed I've even noticed that Windows doesn't install the C compiler be default). So how do people get away with such reckless behaviour if Microsoft's compilers are so grossly incompatible between versions?

I've hacked the code of Net::SSLeay so that it doesn't attempt to pass information from the XS code to the Perl via errno and if I get it working I may submit a patch but I can't help wondering if I'm doing the wrong thing.