To reproduce this use the following:
1. create test.cgi file
------------------------------------
#!/bin/sh
echo 'Content-type: text/html'
echo
echo 'MAIL SAVED'
exec /bin/cat >/dev/null
------------------------------------

Note the cgi must read data from stdin,
(like in POST method).
This is what "exec /bin/cat >/dev/null" for.

then start stunnell
/usr/sbin/stunnel -f -D 7 -d 443 -r 80

Create two files t1 and t2 and use curl compiled with SSL

perl -e 'for(my $x=0;$x<128288;$x++){print " ";}' > /tmp/t1
then
./src/curl -3 -v -d @/tmp/t1 https://127.0.0.1/cgi-bin/test.cgi
hangs.

then do
perl -e 'for(my $x=0;$x<1048576;$x++){print " ";}' > /tmp/t2
./src/curl -3  -d @/tmp/t2 https://127.0.0.1/cgi-bin/test.cgi

and it works. (note that 1048576=2^20)

in the same time forcing curl to use SSL2 works OK
Then access it from curl
./src/curl -3 -v -d @/tmp/jjjXX https://127.0.0.1/cgi-bin/test.cgi

Also stunnel to stunnel work OK when compiled as default,
but It DOES NOT work if you set different buffer sizes in client and server.
To repoduce use:

1. Use standard stunnell (buffer 8192) as a server
/usr/sbin/stunnel -f -D 7 -p /usr/share/ssl/certs/stunnel.pem -d 443 -r 80

2. Make another copy , edit file ssl.c
and put there
/* I/O buffer size */
#define BUFFSIZE      16413 /* 8192 */
then start stunnel
/my/special/anotherbuffersize/stunnel -f -D 7 -c -d 127.0.0.1:97 -r
127.0.0.1:443

3. now access  http://127.0.0.1:97/cgi-bin/test.cgi
(note port is 97 , as set in stunnel client)

./src/curl -3  -d @/tmp/t1 http://127.0.0.1:97/cgi-bin/test.cgi
or, better,
strace  ./src/curl -3  -d @/tmp/t1 http://127.0.0.1:97/cgi-bin/test.cgi
curl does not use any SSL any more, plain http access,
it goes via first stunnel (as a client) to second ( as a server) to plain http.

And it hanges. exactly in the same way as curl via ssl.
even curl strace is similar.

Also note that focing curl to SSL ver2 

./src/curl -2 -v -d @/tmp/jjjXX https://127.0.0.1/cgi-bin/test.cgi
works OK without any hang.

It's fixed in stunnel versions >=3.9.

Also, the stunnel from updates stunnel-3.9-1 
has a bug when started as 
/usr/sbin/stunnel -d 465 -r remote_host_ip:25
It prints 
Dec 20 14:38:16 localhost stunnel[20230]: stunnel 3.9 on i386-redhat-linux-gnu
PTHREAD+LIBWRAP
Dec 20 14:38:16 localhost stunnel[20231]: Cannot create pid file
/var/stunnel/stunnel.209.10.98.90.25.pid
Dec 20 14:38:16 localhost stunnel[20231]: Create: No such file or directory (2)

See, there is a string /var/stunnel in /usr/sbin/stunnel 
which is used in path.
rpm -q stunnel
stunnel-3.9-1
[root@localhost /root]# strings /usr/sbin/stunnel |grep /var/stunn
/var/stunnel/


I am posting this to openssl because there is no stunnel in the packages list

We're working on a stunnel-3.9-2 errata to fix this last bug, and another.

The errata package will be 3.10-2.