Perl's regular expression substitution function is useful for CGI string manipulation, but using the default delimiter / can lead to leaning-toothpick syndrome if you use it for paths or URLs. Consider this line:
Each / must be escaped, leading to an unreadable regular expression. But you don't need to use the default delimiter; in fact, you can use any character you want.
Using ! as a delimiter makes this string substitution much easier to read, and much more likely to work the first time around.