Refine the changes to perlintro.pod after feedback and discussion.

Following useful feedback and suggestions from Neil Bowers, Jason McIntosh
and Dan Book, try to make the sentances shorter and clearer. Hopefully also
avoid some potential abmiguities about "special variables" not just being
all-punctuation or scalars.

pod/perlintro.pod

@@ -161,9 +161,8 @@ A scalar represents a single value:
  my $answer = 42;
 
 Scalar values can be strings, integers or floating point numbers, and Perl
-will automatically convert between them as required.  There is no need
-to pre-declare your variable types - just declare their names using
-the C<my> keyword the first time you use them.  (This is one of the
+will automatically convert between them as required.  You have to declare
+them using the C<my> keyword the first time you use them.  (This is one of the
 requirements of C<use strict;>.)
 
 Scalar values can be used in various ways:
@@ -172,9 +171,8 @@ Scalar values can be used in various ways:
  print "The animal is $animal\n";
  print "The square of $answer is ", $answer * $answer, "\n";
 
-There are a number of "magic" scalars with names that use punctuation symbols,
-and a few that are all uppercase letters.
-These special variables are used for all
+Perl defines a number of special scalars with short names, often single
+punctuation marks or digits. These variables are used for all
 kinds of purposes, and are documented in L<perlvar>.  The only one you
 need to know about for now is C<$_> which is the "default variable".
 It's used as the default argument to a number of functions in Perl, and