Thanks for your comments.
"I think a syntax exiftool -tag1=$tag2 a.jpg would be much more intuitive..."
But then how would you set the value of tag1 to the string "$tag2" rather than the value of tag2?
"I think it would be terrific if exiftool had built string manipulation functions..."
I agree that these would be useful, but I haven't been able to think of a simple
way to add them. My goal is to keep the exiftool command-line interface as
simple as possible (it is already way too complex!), since it is always possible
(and usually fairly simple) to do what you want in Perl and use the ExifTool API
instead of the command-line interface.