<p>Perforce allows people to check in unchanged files. Why any version control system would allow this is beyond me, but that's a topic for another question.  I want to create a trigger that will deny the submission of unchanged files.  However, I have no experience with Perforce triggers.  From what I've read, I'm guessing this would be a "Change-content" trigger since the files being submitted would have to be diffed against the respective head revisions they are about to replace.  I would need to iterate over the incoming files and make sure they had all indeed changed.  The problem is, I have no idea how to go about it.</p>

<p>Can anyone with Perforce trigger experience offer an example or at least point me in the right direction?</p>

If you look at the Triggers table in Perforce, you will see that triggers are nothing but scripts that get invoked when some kind of event happens. In your case, the change-content event is triggered.
You have several options to write scripts that interact with Perforce. The
Perforce downloads page
has libraries and modules for many widely use languages. Any of this will help you and greatly simplify what you need to do. Also, check the
Perforce Documentation page
and download the administrator's guide. It will explain how to create the trigger, etc.
Basically, you need to write a script that will get the information from the change list that is being submitted and for each file in it run a "diff" command against the server. If you find a file that has not change, you need to invalidate the submission.
The Perforce module on you favorite language and the administrators guide will give you all the answers you need.