For reasons of security, this construct is for-
                 bidden if the regular expression involves run-
                 time interpolation of variables, unless the per-
                 ilous "use re 'eval'" pragma has been used (see
                 the re manpage), or the variables contain
                 results of "qr//" operator (see the
                 qr/STRING/imosx entry in the perlop manpage).

                 This restriction is because of the wide-spread
                 and remarkably convenient custom of using run-
                 time determined strings as patterns.  For exam-
                 ple:

                     $re = <>;
                     chomp $re;
                     $string =~ /$re/;

                 Before Perl knew how to execute interpolated
                 code within a pattern, this operation was com-
                 pletely safe from a security point of view,
                 although it could raise an exception from an
                 illegal pattern.  If you turn on the "use re
                 'eval'", though, it is no longer secure, so you
                 should only do so if you are also using taint
                 checking.  Better yet, use the carefully con-
                 strained evaluation within a Safe module.  See
                 the perlsec manpage for details about both these
                 mechanisms.