perlintro: Define metacharacter before using the term

This adds a bit of text about metacharacters that was missing from this
introductory pod.

Author: khwilliamson

pod/perlintro.pod

@@ -584,10 +584,33 @@ the meantime, here's a quick cheat sheet:
  ^                   start of string
  $                   end of string
 
-Quantifiers can be used to specify how many of the previous thing you
-want to match on, where "thing" means either a literal character, one
-of the metacharacters listed above, or a group of characters or
-metacharacters in parentheses.
+Note that in the above, C<$> doesn't mean a dollar sign.  Similarly
+C<.>, C<\>, C<[>, C<]>, C<(>, C<)>, and C<^> don't mean the characters
+you might expect.  These are called "metacharacters".  In contrast, the
+characters C<a>, C<e>, C<i>, C<o>, and C<u> for example, are not
+metacharacters.  They match themselves literally.  Metacharacters
+normally have a meaning that isn't their literal value.  There are a few
+more metacharacters than the ones above.  Below some quantifier ones are
+given.  (The full list is in L<perlre/Metacharacters>).
+
+To make a metacharacter mean its literal value, you "escape" (or "quote")
+it, by preceding it with a backslash.  C<\$> does mean a dollar sign,
+and C<\\> means a literal backslash.
+
+Note also that above, the string C<\s>, for example, doesn't mean a
+backslash followed by the letter C<s>.  In this case, preceding a
+non-metacharacter with a backslash turns it into something that doesn't
+mean its literal value.  Such a sequence is called an "escape sequence".
+L<perlrebackslash> documents all of the current ones.
+
+Perhaps paradoxically, escaping a metacharacter (by preceding it with a
+backslash) turns off it's normal meaning; while escaping a
+non-metacharacter creates an escape sequence that has a meta meaning.
+
+You can specify how many of the previous thing you want to match on by
+using quantifiers (where "thing" means one of: a literal character, one
+of the constructs listed above, or a group of either of them in
+parentheses).
 
  *                   zero or more of the previous thing
  +                   one or more of the previous thing