My hunch is that there's something in your code (which you haven't provided) that's causing the problem. I believe the following Inline::C script demonstrates that we can pass strings (in both directions) via the stack that are considerably larger than 64kb. At least that's what it demonstrates on my Win32 box - as the script ouptputs "8000000".

If the same script produces the same output on your box, then I think we can conclude that the problem you're facing is something other than a stack size limit.

use warnings;
use strict;

use Inline C => Config =>
    BUILD_NOISY => 1;

use Inline C => <<'EOC';

void foo(char * htmFile, int len) {
     dXSARGS;
     sp = mark;
     XPUSHs(sv_2mortal(newSVpv(htmFile, len)));
     PUTBACK;
     XSRETURN(1);
}

EOC

my $len = 8000000;
my $z = 'z' x $len;

my $copy = foo($z, $len);

print length($copy), "\n";
[download]

Cheers,
Rob