This is ugly, but it works. (For 99.9% of all cases - if you are able to predict a scalar reference's stringification, this will fail. :)

sub is_qr {
    my ($qr) = @_;
    return 0 unless ref $qr;
    return 0 if defined $$qr; # Update: added this line.
    return 1 if ref $qr eq 'Regexp';
    return index(qr/$qr/, "$qr") == -1;
}
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