I'm having this exact problem, and none of the suggestions in this thread have helped. I'm also having a fight with curl. Would someone who has curl working properly give this a shot?

curl -d '__VIEWSTATE=dDw1MzgxO3Q8O2w8aTwwPjs\x2BO2w8dDw7bDxpPDEzPjs\x2
+BO2w8dDw7bDxpPDk\x2BOz47bDx0PDtsPGk8OT47PjtsPHQ8O2w8aTw3Pjs\x2BO2w8dD
+w7bDxpPDA\x2BOz47bDx0PDtsPGk8MD47PjtsPHQ8O2w8aTwxPjs\x2BO2w8dDw7bDxpP
+DI\x2BO2k8Mz47aTw0PjtpPDU\x2BO2k8Nj47aTw3PjtpPDg\x2BO2k8OT47aTwxMD47P
+jtsPHQ8cDxwPGw8X2N0bDVfX2N0bDNfX2N0bDBfRm9ybUl0ZW1fMjA1MTA3NF9SZXF1aX
+JlZEZpZWxkVmFsaWRhdG9yX19jdGw1X19jdGwzX19jdGwwX0Zvcm1JdGVtXzIwNTEwNzR
+fTGFiZWxfRm9yZUNvbG9yO0Rpc3BsYXk7PjtsPDI8XGU\x2BO1N5c3RlbS5XZWIuVUkuV
+2ViQ29udHJvbHMuVmFsaWRhdG9yRGlzcGxheSwgU3lzdGVtLldlYiwgVmVyc2lvbj0xLj
+AuNTAwMC4wLCBDdWx0dXJlPW5ldXRyYWwsIFB1YmxpY0tleVRva2VuPWIwM2Y1ZjdmMTF
+kNTBhM2E8Tm9uZT47Pj47Pjs7Pjt0PHA8cDxsPF9jdGw1X19jdGwzX19jdGwwX0Zvcm1J
+dGVtXzIwNTEwNzVfUmVxdWlyZWRGaWVsZFZhbGlkYXRvcl9fY3RsNV9fY3RsM19fY3RsM
+F9Gb3JtSXRlbV8yMDUxMDc1X0xhYmVsX0ZvcmVDb2xvcjtEaXNwbGF5Oz47bDwyPFxlPj
+s1MDxOb25lPjs\x2BPjs\x2BOzs\x2BO3Q8cDxwPGw8X2N0bDVfX2N0bDNfX2N0bDBfRm
+9ybUl0ZW1fMjA1MTA3Nl9SZXF1aXJlZEZpZWxkVmFsaWRhdG9yX19jdGw1X19jdGwzX19
+jdGwwX0Zvcm1JdGVtXzIwNTEwNzZfTGFiZWxfRm9yZUNvbG9yO0Rpc3BsYXk7PjtsPDI8
+XGU\x2BOzUwPE5vbmU\x2BOz4\x2BOz47Oz47dDxwPHA8bDxfY3RsNV9fY3RsM19fY3Rs
+MF9Gb3JtSXRlbV8yMDUxMDc3X1JlcXVpcmVkRmllbGRWYWxpZGF0b3JfX2N0bDVfX2N0b
+DNfX2N0bDBfRm9ybUl0ZW1fMjA1MTA3N19MYWJlbF9Gb3JlQ29sb3I7RGlzcGxheTs\x2
+BO2w8MjxcZT47NTA8Tm9uZT47Pj47Pjs7Pjt0PHA8cDxsPF9jdGw1X19jdGwzX19jdGww
+X0Zvcm1JdGVtXzIwNTEwNzhfUmVxdWlyZWRGaWVsZFZhbGlkYXRvcl9fY3RsNV9fY3RsM
+19fY3RsMF9Gb3JtSXRlbV8yMDUxMDc4X0xhYmVsX0ZvcmVDb2xvcjtEaXNwbGF5Oz47bD
+wyPFxlPjs1MDxOb25lPjs\x2BPjs\x2BOzs\x2BO3Q8cDxwPGw8X2N0bDVfX2N0bDNfX2
+N0bDBfRm9ybUl0ZW1fMjA1MTA3OV9SZWd1bGFyRXhwcmVzc2lvblZhbGlkYXRvcl9fY3R
+sNV9fY3RsM19fY3RsMF9Gb3JtSXRlbV8yMDUxMDc5X0xhYmVsX0ZvcmVDb2xvcjtEaXNw
+bGF5Oz47bDwyPFxlPjs1MDxOb25lPjs\x2BPjs\x2BOzs\x2BO3Q8cDxwPGw8X2N0bDVf
+X2N0bDNfX2N0bDBfRm9ybUl0ZW1fMjA1MTA3OV9SZXF1aXJlZEZpZWxkVmFsaWRhdG9yX
+19jdGw1X19jdGwzX19jdGwwX0Zvcm1JdGVtXzIwNTEwNzlfTGFiZWxfRm9yZUNvbG9yO0
+Rpc3BsYXk7PjtsPDI8XGU\x2BOzUwPE5vbmU\x2BOz4\x2BOz47Oz47dDxwPHA8bDxfY3
+RsNV9fY3RsM19fY3RsMF9Gb3JtSXRlbV8yMDUxMDgwX1JlZ3VsYXJFeHByZXNzaW9uVmF
+saWRhdG9yX19jdGw1X19jdGwzX19jdGwwX0Zvcm1JdGVtXzIwNTEwODBfTGFiZWxfRm9y
+ZUNvbG9yO0Rpc3BsYXk7PjtsPDI8XGU\x2BOzUwPE5vbmU\x2BOz4\x2BOz47Oz47dDxw
+PHA8bDxfY3RsNV9fY3RsM19fY3RsMF9Gb3JtSXRlbV8yMDUxMDgxX1JlZ3VsYXJFeHByZ
+XNzaW9uVmFsaWRhdG9yX19jdGw1X19jdGwzX19jdGwwX0Zvcm1JdGVtXzIwNTEwODFfTG
+FiZWxfRm9yZUNvbG9yO0Rpc3BsYXk7PjtsPDI8XGU\x2BOzUwPE5vbmU\x2BOz4\x2BOz
+47Oz47Pj47Pj47Pj47Pj47Pj47Pj47Pj47Pj47Pj47PndIUNKupgylSQgKnJm59RrmMvd
+T' -d '_ctl5\x3A_ctl3\x3A_ctl0\x3AFormItem_2051074=First' -d '_ctl5\x
+3A_ctl3\x3A_ctl0\x3AFormItem_2051075=Last' -d '_ctl5\x3A_ctl3\x3A_ctl
+0\x3AFormItem_2051076=Address' -d '_ctl5\x3A_ctl3\x3A_ctl0\x3AFormIte
+m_2051077=City' -d '_ctl5\x3A_ctl3\x3A_ctl0\x3AFormItem_2051078=State
+' -d '_ctl5\x3A_ctl3\x3A_ctl0\x3AFormItem_2051079=90210' -d '_ctl5\x3
+A_ctl3\x3A_ctl0\x3AFormItem_2051080=joe@netcom.com' -d '_ctl5\x3A_ctl
+3\x3A_ctl0\x3AFormItem_2051081=213-312-2132' -d '_ctl5\x3A_ctl3\x3A_c
+tl0\x3AFormItem_2051082=Color' -d '_ctl5\x3A_ctl3\x3A_ctl0\x3AFormIte
+m_2051083=Animal' -d '_ctl5\x3A_ctl3\x3A_ctl0\x3AFormItem_2051084=E-m
+ail' -d '_ctl5\x3A_ctl3\x3A_ctl0\x3AFormItem_2051085="New Car"' -d '_
+ctl5\x3A_ctl3\x3A_ctl0\x3AFormItem_2051086="MINI Cooper"' -d '_ctl5\x
+3A_ctl3\x3A_ctl0\x3AFormItem_2051087=Message' -d '_ctl5\x3A_ctl3\x3A_
+ctl0\x3AForm_121155_Submit=Submit' --trace out.txt https://secure.web
+makerx.net/Contact-Us.aspx?hostname=billjacobsmini.com
[download]

When I run it, I get "curl: (35) SSL: couldn't set callback!". I saved off the html from the website in question, and modified the post method to post to my local webserver, which was running the CGI app from the LWP book, which showed all the stuff being POSTed. Yes, they actually USE that crappy long string, and it's always the same. When I try to POST to the real site, I get "500 Internal Server Error". ANY help is appreciated!