A transformation of a string or a number into another string or number is common, BUT the transformation of refs is impossible.
Hence it's far more likely that legacy code will break if something like $_++ became illegal for an aliased input like 3+4 .
Same code for refs wouldn't make sense at all.
I can't come up with more intuitive examples, but I'm sure those exist.
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery