If you need to see the output of headers from the CGI you are writing to make sure they conform to your expectations, you can pass your parameters on the command line:

#> perl mytest.cgi username=myname userid=9999
[download]

Your output is what is sent to the client browser, unless your web server re-interprets your content (some do). If you see anything in the header not matching what a HTML Header should look like, that is likely what is breaking your page.