CCFLAGS	=> $devel_cflags . ' ' . $curl{cflags} . $bits,
that needs to be modified to:
CCFLAGS	=> $Config::Config{ccflags} . ' ' . $devel_cflags . ' ' . $curl{cflags} . $bits,

##</code><code>##

--- Curl.xs_orig        2019-11-20 11:44:50 +1100
+++ Curl.xs     2019-11-20 22:55:23 +1100
@@ -17,6 +17,11 @@
 #include "perl.h"
 #include "XSUB.h"

+#ifdef __MINGW32__
+#undef fread
+#undef fwrite
+#endif
+
 #include <curl/curl.h>
 #include <curl/easy.h>
 #include <curl/multi.h>

##</code><code>##

--- Curl_multi.xsh_orig 2019-11-20 11:54:58 +1100
+++ Curl_multi.xsh      2019-11-20 11:55:35 +1100
@@ -564,8 +564,8 @@
 int
 socket_action( multi, sockfd=CURL_SOCKET_BAD, ev_bitmask=0 )
        Net::Curl::Multi multi
-       int sockfd
-       int ev_bitmask
+       IV sockfd
+       IV ev_bitmask
        PREINIT:
                int remaining;
                CURLMcode ret;