The installation page at
http://imager.perl.org/docs/Imager/Install.html
says
Show quoted text> Imager::File::JPEG uses libjpeg for JPEG image file support.
>
> You may also use libjpeg-turbo.
It doesn't say _how_ to use an alternative JPEG library.
In particular, I'd like to be able to use it with the mozjpeg library
(which supposedly has the same interface).