So I swapped each of the three system() calls for backticks like the following...

# Extract main (left) image.
my $fdbk = qx|exiftool -trailer:all= $mpo -o $dir_in/L.jpg 2>&1 1>/dev
+/null|; 
print "ExifTool error on image 1: $fdbk\n" if $fdbk;
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...and no error prints from the print() command but nevertheless I get three prints of just plain...

"The system cannot find the path specified"

...in the CLI all by themselves. And the program still works. What am I failing to understand still yet?