The answer to your question will depend on what cgi_app.pl does, which we dont' know.

It looks like it's a wrapper of some kind that's calling other scripts, and in some situations it doesn't know what to call. There's a variable that's supposed to hold the name of the script to execute, and whoever wrote cgi_app.pl gave it the value of the string 'undefined' for situations in which it is undefined (aka undef). This would stop it blowing weird errors and possibly dying, but at the expense that you see this odd, somewhat cryptic, behavior...

The fact that it's calling scripts based on user input fills me with fear that it's not very secure. Make sure that users across the internet cannot execute arbitrary commands.

--Pileofrogs