Okay, so now I change the calls to be like so...

# Extract main (left) image.
qx|exiftool -trailer:all= $mpo -o L.jpg 2>&1 1>/dev/null|; 
print "ExifTool error on image 1: $?\n" if $?;
[download]

...and now get feedbacks of...

The system cannot find the path specified.
ExifTool error on image 1: 256
[download]

...and yet STILL the program works!

PS:

Admittedly my present job has kept me long away from Perl. I use it but seldom now and so have grown rusty. Once I had used to sometimes answer for other people in the same way my own questions had once upon a time used to get answered. When I knew how to do it, I would answer thus, saying...

It will work if you do it like so. Here's a simplified example.

Basic courtesy that would be...or so seems to me. Perhaps I'm suffering a false nostalgia but having my ignorance (which is presumed in posing any question) rubbed in leaves me to feel as if the culture may have changed here?