I was skeptical about this warning, and a simple experiment shows that it is not accurate:

  DB<1> $x = '???\E???'
 
  DB<2> p "\Q$x"
\?\?\?\\E\?\?\?
  DB<3>
[download]

A \Q cannot be ended by a \E in an interpolated string. And, using B::Deparse's -q option in 5.6, we can see why:

~> perl -MO=Deparse,-q -e '$x = q{?\E?}; $y = "a\Q$x\Eb"; print "$y\n"
+'
$x = '?\\E?';
$y = quotemeta $x;
print $y . "\n";
-e syntax OK
[download]

This shows that \Q\E inside a double-quoted string is actually compiled as concatenation and quotemeta. The value of $x at runtime won't change the scope of the quotemeta().