I wonder who has THE solution which combines shortness and elegance in a tremendous way

You do realise that it is not always possible to reconcile these two aims?

A single "one size fits all" test will not give you the chance of reporting why the password is no good. If this is not a problem, then my best shot looks like:

#! /usr/local/bin/perl -w

use strict;

while( <DATA> ) {
    chomp;
    my $ok = (
            length $_ >= 5
        and length $_ <=8
        and /\d/
        and /[A-Z]/
        and /[a-z]/
    );
    print sprintf('%-12s', $_), ($ok ? 'valid' : 'invalid '), "\n";
}
[download]

I would probably break it up into separate test as shown elsewhere in this thread. You might also want to avoid reinventing the wheel and look at Data::Password::BasicCheck. Also, given that most people never bother to change the password they are given, you should also investigate Crypt::GeneratePassword.