sub foo {
  my ($tmp, @list1, @list2, %vals) = ('aaaaa');
  @list1 = generate_list1();
  @list2 = generate_list2();

  if (@list1 > @list2) {
    push @list2, $tmp++ for 1..@list1-@list2;
  } else {
    push @list1, $tmp++ for 1..@list2-@list1;
  }

  @vals{@list1} = @list2;

  return %vals;
}

my %tmp = foo();
my @list1 = sort grep !/^aaa/, keys %tmp;
my @list2 = sort grep !/^aaa/, values %tmp;