Problem in "index" function in recognizing repeating substrings

[monkfan]

Most revered monks,
The built-in index function seems to be able to recognize only the first substring given two identical substrings from a string. My code below:

#!/usr/bin/perl -w
use strict;

my $str = 'NNNNNATCGNNNNATCG';
my @substr  = qw(ATCG ATCG);
foreach (@substr)
{
   my $id = index($str,$_);
   print "$id\n"; 
}

Returns:

5
5

How can I overcome this problem such that the index function can recognize the second substrings in the correct position? Returning this instead:

5
13