#!/usr/bin/perl

use strict;
use warnings;

my $length  = 1;
my $max     = 1;
my @strings;
for ( 1 .. $max ) {
    push @strings, rand_nums($length);
}
print "@strings", "\n";

{
my %cache; 

sub rand_nums {
    my %local_cache;  
    my ($length) = 8; 
    my $lower    = 4;
    my $upper    = 21;   
    my $serial = int( rand($upper - $lower) ) + $lower;  
        $local_cache{$serial} = 1;  
        for ( 2 .. $length ) {
            my $num = int( rand($upper - $lower) ) + $lower;    
            redo if exists $local_cache{$num};
            $local_cache{$num} = 1;  
            $serial .= "-$num";      
        }
        rand_nums($length) if exists $cache{$serial};    
        $cache{$serial} = 1; 
        return $serial;
    }
}