sub pin {
    srand;
    my $pin;
    my $i = 0;
    while ( $i++ < 4 ) {
        $pin .= int( rand(9) );
    }
    return $pin;
}

##</code><code>##

my @counterexamples;
my %digits = map { $_ => 1 } 0 .. 9;
for ( 1 .. 10000 ) {
    my $pin = pin();
    if ( $pin !~ /^\d\d\d\d$/ ) {
        push @counterexamples => $pin;
    }
    if (%digits) {
        foreach my $digit ( keys %digits ) {
            if ( $pin =~ /$digit/ ) {
                delete $digits{$digit};
            }
        }
    }
}
ok !@counterexamples, 'All pin numbers should be four digits'
    or diag +Data::Dumper->Dump( [ \@counterexamples ], ['*bad_pins'] );
ok !%digits, '... and all digits should be used'
    or diag sprintf "Unused digits are '%s'", join ', ', sort keys %digits;

##</code><code>##

sub pin {
    return sprintf "%04d" => int rand 10000;
}