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 )  );