Another option is to use the same approach as find -exec.

prog [options] --ffmpeg [ffmpeg args] [sentinel] [options] [args]
[download]

I'm not a fan of this approach because of its down sides.

• Confusing. Options to your program and options for ffmpeg aren't distinguishable at a glance.
• Requires coming up with a sentinel value.^[1]
• No support for this in Getopt::Long (as far as I know).

Footnotes:

1. ";" and "+" serve as sentinels for POSIX find -exec. ";" must be escaped since it already has meaning to the shell, though.