The preface will explain how much Perl
the reader is expected to know, right?. Given the topic, I suggest
that you assume they haven't worked with Perl regular
expressions at all, in which case you should explain the
s and
m operators before you use
them.
You don't have to spend a lot of time at it, but when I see
"Learning..." in the title of a book, I expect the basics
of the subject matter to be covered early, and re-emphasized
in new sections (or new contexts). In chapter 2
you could say,
Perl's m operator is used to match patterns
in a string, such as
m/Hello, world!/
The / marks delimit the pattern; any of Perl's quoting or
bracketting characters can be used as well, though the /
is the most traditional, and in fact the m operator
can be left out if you use them.