None of those do what was asked for. larryk's fine suggestion of (Golf) Ordered Combinations does what was asked for but requires that all 1,048,576 ten-character strings be stored in memory for you to get the answer. With Perl arrays, this takes several hundred MB (in my quick test) and so could be a real problem. If you just write the combinations to a file, then you end up with about a 10MB file, which isn't a big deal.

So here is a quick hack to do that:

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;
[download]

Update: And here is a context-free iterator solution:

sub count {
    my( $alphabet, $seq )= @_;
    my $base= length($alphabet);
    my $pos= 0;
    my $val;
    while(  $base <= ( $val= 1 + index( $alphabet, substr($seq,$pos,1)
+ ) )  ) {
        substr( $seq, $pos, 1 )= substr( $alphabet, 0, 1 );
        return   if  length($seq) <= ++$pos;
    }
    substr( $seq, $pos, 1 )= substr( $alphabet, $val, 1 );
    return $seq;
}

my( $len, $alphabet )= @ARGV;
my $seq= substr($alphabet,0,1) x $len;
do {
    print $seq, $/;
} while(  $seq= count( $alphabet, $seq )  );
[download]