You're right, this conflicts with what my copy of Programming Perl (2nd ed.) says, on page 358:

The only way to bypass the tainting mechanism is by referencing subpattern variables set by an earlier regular expression match.

and with the perlsec manpage:

The only way to bypass the tainting mechanism is by referenc­ing subpatterns from a regular expression match.

And I see the same behavior on my system (which is also Perl 5.6.1 on RedHat Linux 7.x (7.2 instead of 7.1)).