UPDATE: I've added two more tests to .t/bo/Rekening.t to check the protected setCreditAmount and setDebetAmount subs (and of course changed the number of tests from 13 to 15

Thanks for the comments and answers :)

I think I'll go with the following (sorry for the dutch names, Passief eq Passive Account and Rekening eq Account (the base class):

I'll implement the wrapper class for testing protected subs later on :)

./t/bo/Passief.t

#!/usr/bin/perl

use strict;
use warnings;

use Math::BigInt;

use Test::More tests => 12;
use Test::Exception;

# We need the Rekening and Actief classes
use src::bo::Passief;

# we test the passief account
my $passief = Passief->new("kapitaal");
is( ref $passief, "Passief", "A passief object" );
is( $passief->getName(), "kapitaal", "It's kapitaal" );

lives_ok { $passief->addToAccount() } "addToAccount is overwritten";
$passief->addToAccount( Math::BigInt->new(5) );
is( $passief->getDebetAmount(), Math::BigInt->new(5), "kapitaal has 5 
+credit" );
is( ref $passief->getDebetAmount(),
    "Math::BigInt", "getCreditAmount() returns a BigInt" );

lives_ok { $passief->subtractFromAccount() }
  "subtractFromAccount is overwritten";
$passief->subtractFromAccount( Math::BigInt->new(4) );
is( $passief->getCreditAmount(), Math::BigInt->new(4), "kapitaal has 4
+ debet" );
is( ref $passief->getCreditAmount(),
    "Math::BigInt", "getDebetAmount() returns a BigInt" );

is( $passief->balance(), 1, "kapitaal has a 1 balance" );
is( ref $passief->balance(), "Math::BigInt", "balance() returns a BigI
+nt" );

# we test if we can call setCreditAmount directly
throws_ok { $passief->setCreditAmount(0) } qr/setCreditAmount is prote
+cted/,
  "setCreditAmount is protected";

# we test if we can call setDebetAmount directly
throws_ok { $passief->setDebetAmount(0) } qr/setDebetAmount is protect
+ed/,
  "setDebetAmount is protected";
[download]

./t/bo/Rekening.t

#!/usr/bin/perl

use strict;
use warnings;

use Math::BigInt;

use Test::More tests => 15;
use Test::Exception;

# We need the Rekening and Actief classes
use src::bo::Rekening;

# a quick test to see if rekening works
my $rekening = Rekening->new("rekeningnaam");
is( ref $rekening, "Rekening", "A rekening object" );

is( $rekening->getName(), "rekeningnaam", "It's rekeningnaam" );

my $zero = Math::BigInt->new(0);

is( $rekening->getCreditAmount(), $zero, "kas has 0 credit" );
is( ref $rekening->getCreditAmount(),
    "Math::BigInt", "getCreditAmount() returns a BigInt" );
is( $rekening->getDebetAmount(), $zero, "kas has 0 debet" );
is( ref $rekening->getDebetAmount(),
    "Math::BigInt", "getDebetAmount() returns a BigInt" );

# we test a rekening without a name
my $rekening2 = Rekening->new();
is( ref $rekening2, "Rekening", "A rekening object" );
is( $rekening2->getName(), undef, "It's undef" );

# we test if we can call addToAccount directly
throws_ok { $rekening->addToAccount() } qr/Rekening is a base class/,
  "addToAccount is protected";

# we test if we can call subtractFromAccount directly
throws_ok { $rekening->subtractFromAccount() } qr/Rekening is a base c
+lass/,
  "subtractFromAccount is protected";

# we test if we can call balance directly
throws_ok { $rekening->balance() } qr/Rekening is a base class/,
  "balance is protected";

# we test if we can call setCreditAmount directly
throws_ok { $rekening->setCreditAmount(0) } qr/setCreditAmount is prot
+ected/,
  "setCreditAmount is protected";

# we test if we can call setDebetAmount directly
throws_ok { $rekening->setDebetAmount(0) } qr/setDebetAmount is protec
+ted/,
  "setDebetAmount is protected";

# magic, we test the setCredit protected sub
my $setCredit;
my $setDebet;
{
    package Sub::Rekening;
    our @ISA = 'Rekening';
    eval { 
        $rekening->setCreditAmount($two); 
        $rekening->setDebetAmount($one);
        $setCredit = $rekening->getCreditAmount();
        $setDebet = $rekening->getDebetAmount();
    };
}
is( $setCredit, $two, "setCreditAmount returns 2" );
is( $setDebet, $one, "setDebetAmount returns 1" );
[download]