$ perl -e '"abc" =~ s/c$//'
Can't modify constant item in substitution (s///) at -e line 1, at EOF
Execution of -e aborted due to compilation errors.

##</code><code>##

$ perl -e '2++'
Can't modify constant item in postincrement (++) at -e line 1, near "2++"
Execution of -e aborted due to compilation errors.