Matching an exact string is also a valid regex, and I think the GP's simple attempt to use the substitution syntax illustrates JWZ's point even better than a more complex expression. There are so many ways of screwing it up that even a simple joke contained a bug.
The notation s/<regex1>/<regex2> is perl for "replace every instance matching regex1 by regex2. Literal strings are regexes that match only themselves.
Not just perl. Also sed, which is likely where perl got it from.
Not just sed. Also ed, which is where sed got it from.