Well, let's do what we should do when we have a data structure we don't understand: look at it in the debugger.

$ perl -d xml_sample.pl

Loading DB routines from perl5db.pl version 1.28
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(xml_sample.pl:5):        my $config = XMLin("testFile.xml",forc
+earray => ['person','email']);
                                                                      
+            DB<1> l 
5==>    my $config = XMLin("testFile.xml",forcearray => ['person','ema
+il']);
6:      my @persons = $config->{person};
7:      print "Size:".@persons;
   DB<1> c 6  # run continuously to line 6

main::(xml_sample.pl:6):        my @persons = $config->{person};

    DB<2> x $config
0  HASH(0x1988e88)
   'person' => ARRAY(0x19acb00)
      0  HASH(0x1988f30)
         'email' => ARRAY(0x19acba8)
            0  'joe@smith.com'
            1  'jsmith@yahoo.com'
         'firstname' => 'Joe'
         'lastname' => 'Smith'
      1  HASH(0x198a0cc)
         'email' => ARRAY(0x19acab8)
            0  'bob@smith.com'
         'firstname' => 'Bob'
         'lastname' => 'Smith'
  DB<3> q
$
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So we can see that the value of $config->{person} is actually a reference to an array, not an array itself. You'll have to dereference it to get an actual array. Try @{ $config->{person} }. The elements in the array created by this expression will be the two references to the hashes containing references to arrays containing the email values.