Hi. LWP can be used for the API functionality, and Moose to help building an OO interface.
There is a pre-existing FFmpeg module for Perl, but it is very old (updated 2006) and probably not reliable. It will likely be necessary to either
1. write a custom FFmpeg interface (using ffmpeg's C API), or
2. invoke ffmpeg commands using system() or exec() (a 'shell' interface).
Time permitting, I will try to upload a video demo with a watermark added by ffmpeg. I am placing this initial bid partly as a placeholder - and will update the cost and timeframe if/when I have time to run some tests.
I am based in Ireland - GMT timezone.