Let's see what I can offer to sort this out: first, you did not specify if this was a Web Application or Client/Server, or both, so here is the poop on Send Event and Open URL:
First, Send Event is not Web Compatable; however, Open URL is, obviously. But, Open URL does not open anything other than web pages and email when used from IWP (Don't know about Custom), and for reasons no one seems to have a definitive answer for even though the documentation says that it will. You will notice that anything you try with OpenURL in IWP, other than mailto will append an http:// to the address.
However, in a ClientServer, Open URL works just fine, and so does send event.
For the platform dependant part of you question, let me try this: Filemaker works on both Mac and Windows, so send event will work accordingly depending on what OS the solution is installed under. You may need to have a Windows and Mac version of your solution (I am not 100% sure about this), but it makes sense since the file-level command syntax and calls are very different. So the short answer is that your solution may be platform dependant, however, Filemaker as a development environment is not.
Lastly, two-way data communication might be better accomplished using an ODBC connection to your original tables. I don't know enough about PERL to even begin an answer here.
I am not en expert by any means, and am largely full of crap usually, but hope this is of some help. If for no other reason than to start some dialog with real experts!
Good Luck!