my $j = 0;

for (my $i = 0; $i <= $#array; ++$i) {
  $current = $array[$i];
  if($current == undef) {
    push @empty, $j++;
  }
}

print "undefined indices:  @empty \n";

##</code><code>##

my @empties = ();
for my $i(0..$#array) {
   unless(defined $array[$i]) {
       push @empties, $i;
   }
}
print "undefined indices @empties\n";