Why there's such big difference? I cannot understand it...

(Yeah ... sorry, not much point running Inline::C scripts when we're dealing with something that's specific to Inline::CPP ;-)

I don't grok C++, and I can only assume that in your simplified Inline::CPP script, the write() call in ostream has not been made visible. It's definite that 'write' has been defined to 'PerlLIO_write' - just try to use write() in your Inline::CPP script and you'll be able to verify:

use strict;
use Inline 'CPP' => config =>
     BUILD_NOISY => 1;

foobar();
     
use Inline 'CPP' => <<'CODE';

#include <perl.h> // not needed - gets included automatically
#include <iostream>

void foobar() {
    std::cout << "foobar" << std::endl;
}

void break_me(SV * a, SV * b) {
   write(a, b);  
}

CODE
[download]

That generates the error:

try_pl_dec5.xs:28:14: error: macro "PerlLIO_write" requires 3 argument
+s, but only 2 given
    write(a, b);
[download]

In ostream, the write() declaration is preceded (next line up) by __ostream_type&. I don't know whether that puts aome sort of condition upon whether the function is visible or not.
Maybe if you include the same headers in the compilation of your Inline::CPP script as were used for the C++ script then things might start to add up correctly.

I know it's messy ... XS is like that, but you just have to work through it.

Cheers,
Rob