use strict;
use warnings;

use Tie::File;

die "File does not exist" unless -f $ARGV[0];

tie my @file, 'Tie::File', $ARGV[0] or die "Could not tie file";

if ( grep !/^[actg]+$/i, @file )
{
        print "BAD\n";
}
else
{
        print "OK\n";
}