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)).

In reply to Re: Unexpected de-tainting with hash keys by sgifford
in thread Unexpected de-tainting with hash keys by eskwayrd

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