my $string = "TGAT";
my @nts = map { 
    my $tmp = $string;
    substr $tmp, $_, 1, '.';
    $tmp;
    } (0 .. length ($string) -1);
my $groupings = join '|', @nts;

my $sample = "TGATTGGAATGTTAGAT";

while ( $sample =~ /($groupings)/go )
{
    print "Matched $1 ending at position ", pos $sample, "\n";
}