tye raised some good points as to why it shouldn't be done, and you haven't brought up any reason why it should be done. I once tried to come up with a situation where your proposed fix actually mattered, and didn't come up with anything. If you were looking for undef, you'd want to know more, such as the index at which the undef was found. If so, you'd use
my $idx = first { !defined($a[$_]) || ... } 0..$#a;