under the attribute the w3 spec lists :
method      (GET|POST)

I inferred (ass-u-me-d:) ) that to properly submit data, you should only use one or the other, discounting the idea that someone could read in the url to retrieve GET data. On a side note, I see that CGI.pm says

start_form() will return a <FORM> tag with the optional method, action
+ and form encoding that you specify. The defaults are: 

    method: POST
    action: this script
    enctype: application/x-www-form-urlencoded
[download]

while w3's spec says

method = get|post [CI] 
This attribute specifies which HTTP method will be used to submit the 
+form data set. Possible (case-insensitive) values are "get" (the defa
+ult) and "post". See the section on form submission for usage informa
+tion.
[download]

And find it mildly amusing that the default methods disagree. Which, it seems is often the case for web standards documents.