print "Content-type: $tripmimetype\n";
[download]

A CGI Header needs to end with two newlines (not one). This could be your problem. Can you open your show_ipx.pl in a browser? It should display a download dialog or something like that. Or does it show an error? If so, can you check the web server error log? If the second \n is missing you would see an "incomplete headers" message.