use strict;
use warnings;

my %counts;
my @seq;
local $/='';

while(<DATA>){
        chop;
        s/\n//;
        @seq = split '';
        }

foreach my $element (@seq){
        #count for each DNA base or group foreign letters
        $element =~/[agct]/gi ? $counts{$element}++ : $counts{'N'}++;
        }      

use Data::Dumper;
print Dumper(\%counts);
__DATA__
agcttgtc
agtccxffhhhh