こんなもんで書けてる
CONTENT_TYPE = application/x-www-form-urlencoded
HTTP_ACCEPT = */*
HTTP_ACCEPT_ENCODING = gzip, deflate
HTTP_ACCEPT_LANGUAGE = en
HTTP_CACHE_CONTROL = max-age=259200
HTTP_CONNECTION = Keep-Alive
HTTP_COOKIE = PON=test
HTTP_HOST = i2ch.net
HTTP_PRAGMA = no-cache
HTTP_REFERER = http://i2ch.net/check/index.html
HTTP_USER_AGENT = Mozilla/4.0 (compatible; MSIE 5.5; Windows 98)
QUERY_STRING =
REQUEST_METHOD = POST
REQUEST_URI = /test/bbs.cgi

ダメだったのは↓だった
HTTP_CONNECTION = keep-alive
HTTP_USER_AGENT = Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 7.0)