use warnings;
use strict;

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

use Inline C => <<'EOC';

void to_FH(FILE * stream) {
     fprintf(stream, "hello");
     fflush(stream);
}

EOC

my ($got1, $got2);

$got1 = get_string_1();
chomp($got1); # just in case ...
print "OK 1\n" if $got1 eq "hello";


$got2 = get_string_2();
chomp($got2); # just in case ...
print "OK 2\n" if $got2 eq "hello";


sub get_string_1 {
   # Write to a temporary file
   open TEMPFILE, '+>', undef or die $!;
   to_FH(*TEMPFILE);
   seek TEMPFILE, 0, 0;
   my $ret = <TEMPFILE>;
   close TEMPFILE;
   return $ret;
}


sub get_string_2 {
   # Write to a memory file
   my $out;
   open MEM, '+>', \$out or die $!;
   to_FH(*MEM);
   #close MEM; # Makes no difference
   return $out;  # returns undef ... why ?
}

__END__
For me, outputs:
OK 1
Use of uninitialized value $got2 in scalar chomp at try.pl line 24.
Use of uninitialized value $got2 in string eq at try.pl line 25.

##</code><code>##

void to_FH(PerlIO * stream) {
     FILE * stdio_stream = PerlIO_exportFILE(stream, NULL);
     fprintf(stdio_stream, "hello");
     fflush(stdio_stream);
     PerlIO_releaseFILE(stream, stdio_stream);
}