#!/usr/bin/perl
use warnings;
use strict;

use constant PWCHARS =>  ['0'..'9', 'A'..'Z', 'a'..'z'];
sub rand_char {
    PWCHARS->[rand @PWCHARS]
}

my $foo;
for (1..( ARGV[0] || 8 ) ) {
    $foo .= rand_char;
}
print $foo, $/;