Individual defaults are no problem.

set my $var= DEFAULT

does it already.

If there is no prior value, it will be undef because of my .

If you need a dedicated Missing different from undef I could return an error object.

Implementation is no problem, I'd need to think about the best API.

There might also be the case of reused variables - like in a loop - were you skipped the my, but don't want to default to the prior value but undef.

set $var

Again only a question of designing a intuitive API/Syntax and testing use cases.

°) my defaults to undef