It is possible to do this. However it would result in impossible to avoid warnings in the common case where someone is sorting a list which has a mix of numbers and strings. This would be annoying enough that you are unlikely to get general agreement on adding your warning. As well it would slow things down because you force cmp to study whether or not strings look like numbers, rather than just going ahead and comparing them.
By contrast the warning you are comparing it to will catch any time when Perl has to use a string as a number and doesn't think that it looks like a number. It shows up for any numerical operator, not just <=>. This warning is far more useful because if it shows up, you almost certainly are doing something wrong. And the performance penalty is negligable because the operation that you have to do to test for the error is an operation that you are already doing (namely test the string to see if it has any reasonable way of being interpreted as a number).
If you still want to argue for it, use the perlbug utility to send your suggestion to the core Perl developers. Do not be dismayed if they reject it.