perlsec: minor taint edit

Paragraph signalling option to disable taint landed in the middle
of another sentence, breaking it.

pod/perlsec.pod

@@ -45,7 +45,13 @@ these.  Other checks, however, are best supported by the language itself,
 and it is these checks especially that contribute to making a set-id Perl
 program more secure than the corresponding C program.
 
-You may not use data derived from outside your program to affect
+Support for taint checks adds an overhead to all Perl programs,
+whether or not you're using the taint features.
+Perl 5.18 introduced C preprocessor symbols that can
+be used to disable the taint features.
+
+When taint is enabled,
+you may not use data derived from outside your program to affect
 something else outside your program--at least, not by accident.  All
 command line arguments, environment variables, locale information (see
 L<perllocale>), results of certain system calls (C<readdir()>,
@@ -56,11 +62,6 @@ Tainted data may not be used directly or indirectly in any command
 that invokes a sub-shell, nor in any command that modifies files,
 directories, or processes, B<with the following exceptions>:
 
-Support for taint checks adds an overhead to all Perl programs,
-whether or not you're using the taint features.
-Perl 5.18 introduced C preprocessor symbols that can
-be used to disable the taint features.
-
 =over 4
 
 =item *