use strict;
use warnings;

my @strs = qw(GNNTCGANNTT GAATCGNNNTT GANNCGNNNNN);

for my $str (@strs) {
    my $rstr = reverse $str;
    my @starts;

    push @starts, $-[1] while $rstr =~ /(N+)/g;
    
    if (1 == @starts) {
        print "$str: One N found at @starts\n";
    } elsif (@starts) {
        print "$str: Ns found at @starts\n";
    } else {
        print "No N found in $str\n";
    }
}