Your point is, of course valid. However, you should test your code before running it. Your or is lower precedence than = (see Operator Precedence and Associativity). This is, for example, why the classic or die construct works so well. This also means your code will never actually sort by string - warnings would have told you that you are performing a comparison in void context. In addition, + is a no-op in unary operator context; you likely meant the numification ("Venus") operator, 0+. This will not suppress the string in numeric context warnings either, since you are still using a string in a numeric context.

If you want to do auto-detection, you'd be better off using the Conditional Operator using looks_like_number from Scalar::Util:

sub cmp_by {
    use Scalar::Util 'looks_like_number';
    my $result = 0;
    for my $term (@_) {
        $result ||= looks_like_number($a)
                  ? $a->{$term} <=> $b->{$term}
                  : $a->{$term} cmp $b->{$term};
    }
    return $result;
}
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