I kind of see the problem you perceive, though I'd much prefer to handle it with defined:

foreach my $employee (@employees) {
    if ( defined $employee->salary and $employee->salary < $threshold 
+) {
        increase_salary( $employee, 3_000 );
    }
}
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Double reference to $employee->salary or not; I bet it is a) more efficient than your module; b) far less prone to misunderstanding or accidental disablement than your scary-action-at-a distance module.

Personally; I think that instead of initialising things as unknown, you'd achieve a better effect by initialising them as #NaN>.

Indeed, I can see the case for a pragma to cause undef to be treated as #NaN in numeric contexts.

A lexically scoped use undefAsNaN; could be very useful I think.