Follow this easy sequence of steps:

1. Make sure this works:

   #!/bin/sh
   echo content-type: text/plain
   echo
   date
   [download]

   (If you're not on Unix, you can skip this step, but this reveals a lot about the CGI setup without having to introduce the unknown of Perl.)
2. Make sure this works:

   #!/your/path/to/perl
   print "Content-type: text/plain\n\n";
   print "Hello world at " . localtime() . "\n";
   [download]

3. Now take your original script, and insert these right after the #! line:

   #!/your/path/to/perl
   BEGIN { print "Content-type: text/plain\n\n--OUTPUT--\n"; }
   ... rest of your script ...
   [download]

   and make sure your code passes perl -c.

You should now see as the first few lines of output the exact headers and body sent by your program to the server. If the first lines up to the blank line are not nicely formatted headers, fix your script as needed. Once the headers are fixed, you can remove the BEGIN block.

Also, every time you see that sort of error, a sometimes-useful error message has been written to your web server's error log, so find that as well. If you don't have access to the server log, a simple fix is to change step three's suggestion to:

BEGIN {
  print "Content-type: text/plain\n\n--OUTPUT--\n";
  open STDERR, ">&STDOUT"; # errors go to browser now
}
... rest of your script ...
[download]

And now the error messages show up in your browser, as if your browser was the result of invoking the script as a command line.

-- Randal L. Schwartz, Perl hacker
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