I quite like these suggestions, especially the first one with :zero.

I think that generally a better control over warnings is desirable.

For example if I test if a string has some content I check for defined $str && length $str. It feels clumsy, but a simple $str returns false if the string is "0", and a length $str warns if $str is undef.

In these cases I would want to disable some warnings specifically for a few builtins.

no warnings uninitialized => ['length', 'eq']; would be the p5ish way to express this.