Is there a better way to check for taintedness of a variable, say with a CPAN module (or some internal Perl thing I can check if I create my own XS module)?

The immediate solution that comes to my mind is to use Scalar::Util which belongs to core since 5.7.3 and its tainted function. From the module's documentation:

=item tainted EXPR

Return true if the result of EXPR is tainted

    $taint = tainted("constant");       # false
    $taint = tainted($ENV{PWD});        # true if running under -T
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You may learn more about in the sections of perlsec devoted to tainting. You will find there the expression mentioned by Anno at Re^2: Checking to see if Taint mode is enabled and the recommendation for Scalar::Util::tainted (also mentioned in the same node) if you're using 5.8.0 or later.