Interesting question... will have to try this out.

call_sub;

my $what_do_I_get = sub {
  return undefined;
}
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From The Docs

...You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine, assign to a variable, or pass as a parameter. ...

Not sure if one could make that the default though..?!?