sub count {
    my( $length, $alphabet )= @_;
    my @alphabet= split //, $alphabet;
    my @digit= (0) x $length;
    my $seq= $alphabet[0] x $length;
    while(  1  ) {
        print $seq,$/;
        my $pos= 0;
        while(  @alphabet <= ++$digit[$pos]  ) {
            substr( $seq, $pos, 1 )= $alphabet[ $digit[$pos]= 0 ];
            return if $length <= ++$pos;
        }
        substr( $seq, $pos, 1 )= $alphabet[ $digit[$pos] ];
    }
}

count( @ARGV );
# count( $_, "ACGT" )   for  8..10;