Why `stoutest` is not a valid regular expression?

Question

From perlop:

If "/" is the delimiter then the initial m is optional. With the m you can use any pair of non-whitespace characters as delimiters. This is particularly useful for matching path names that contain "/", to avoid LTS (leaning toothpick syndrome). If "?" is the delimiter, then the match-only-once rule of ?PATTERN? applies. If "'" is the delimiter, no interpolation is performed on the PATTERN. When using a character valid in an identifier, whitespace is required after the m.

So I can pick up any letter as a delimiter. Eventually this regex should be fine:

stoutest

That can be rewritten

s/ou/es/

However it does not seems to work in Perl. Why?

$ perl -e '$_ = qw/ou/; stoutest; print'
ou

Answer 1

Because Perl can't pick out the operator s

perldoc perlop says this

Any non-whitespace delimiter may replace the slashes. Add space after the s when using a character allowed in identifiers.

This program works fine

my $s = 'bout';
$s =~ s toutest;
say $s;

output

best

Answer 2

Because stoutest, or any other string of alphanumeric characters, is a single token in the eyes of the Perl parser. Otherwise we couldn't use any barewords that begin with s (or m, or q, or y).

This works, though

$_ = "ou";
s toutest;
print