#!/usr/bin/perl

use strict;
use warnings;

my @strings = qw(
  GNNTCGANNTT
  GAATCGNNNTT
  GANNCGNNNNN
);

my @found;

for my $string( @strings ){
  my $revstring = reverse $string;
    
  $revstring =~ m<
                    (?:
                        .*?
                        (?{ [ $^R ? @{ $^R } : () , pos ] })
                        NN+
                    )+
                    (?{ @found = @{ $^R }; })
                 >x;

  print "String: $string     Positions: ", join( ', ', @found ), "\n";
}