I'd say that unless you habitually and religiously always assign your functions to individual scalars, eg: my( $x, $y, $z ) = yourfunc();, then there is no way to do this check.

If you assign your function to:

1. an array: my @vals =  yourFunc();
2. or a hash: my %hash = yourFunc();
3. or use it in a for loop: for my $x ( yourFunc() ){
4. or to map;  ... map{ ... } yourFunc();
5. or grep;  ... grep{ ... } yourFunc();
6. or as an arg to any other function: someFunc( 1, 'fred', yourFunc() ); user or built-in;

then there is no way to know how many values it should return.