ActivePerl - The specified CGI application misbehaved by not returning a complete set of HTTP headers.

Today a friend called with a problem. He move this ASP/Perl website to a new server and now it didn't work.

Yes, that's right.. Perl AND Asp. Any theories about the emotional stability of the designer should be thought quietly to oneself. lol..

Loading the process.pl page gave this friendly error message.

Premature end of script headers:
Internal Server Error: 500 internal server error
The specified CGI application misbehaved by not returning a complete set of HTTP headers.

My first thought was the app was throwing some error message back, causing IIS to choke. The problem was that it didn't give us a useful (actionable) error message.

Looking at the program's code, it was writing out to a text file and didn't have permissions to write. We fixed that, but it still failed with the same error.

I looked in the (9,000 line) include.pl file trying to see which modules it was loading. That probably would have worked except the code caused me to briefly lose consciousness. I hate it when that happens.

Google saved me. One of the modules in CGI:: has the ability to return fatal errors to the browser. Here is the magic return-a-useful error incantation.

use CGI::Carp qw(fatalsToBrowser);

As it turns out, the application wanted the Mail::Sendmail module. We installed it and viola, the site worked. Time for a diet coke. Life is good.