The procedural method:
$balance = add_to_acct($balance, $amount);

The OOP method:
my $acct_obj = new BankAccount($initial_balance);
$acct_obj->add_to_acct($amount);

##</code><code>##

add_value($x);
add_value($y);
add_value($z);
my @all_values = get_all_values;

##</code><code>##

@all_values = add_value($x, @all_values); 
# gives the list to add to, and takes the newly added list # as a return value
@all_values = add_value($y, @all_values); 
...
# Now anytime you inspect @all_values, it has the currently # added elements.