For an alternative solution, you can create your own library that provides the free() implementation:

C:\pscrpt> type free.c

#include <malloc.h>

void C_free(void * ptr) {
  free(ptr);
}


C:\pscrpt> gcc -c free.c

C:\pscrpt> ar rcs libC_free.a -o free.o

C:\pscrpt> type try2.pl
use strict;
use warnings;

BEGIN{
use Cwd;
$m::cwd = getcwd()
}

use Inline C => Config =>
    LIBS => "-L$m::cwd -lC_free";

use Inline C => <<'END_OF_C';
#define _GNU_SOURCE
void foo() {
    char *s;
    asprintf(&s, "%s", "foo");
    C_free(s);
}
END_OF_C

foo();

__END__

C:\pscrpt> perl try2.pl

C:\pscrpt>
[download]

The success of that script (try2.pl) relies on it being in the same directory as libC_free.a.
Otherwise, modify the LIBS arg in the Inline Config section as necessary.

Cheers,
Rob