my $dna_input = $ARGV[0]; # turn command line input into a variable

my @dna = split("", $dna_input); #split the variable into an array so as to check each nucleotide

my $count = 0; # keep track of position so as to report where there is a non-nuceloetide character
my $base;
foreach $base (@dna) {
        if($base eq 'A' || 'T' || 'C' || 'G') {
                $count += 1;
                }
        else {
                 print "Non-nucleotide at $count position\n";
                }
}