Another way to do it ...

package Simple;

use Class::MethodMaker
    get_set         =>  [ qw/ a b / ],
    new_with_init   =>  'new';

sub init {
    my ($class, %args) = @_;

    my %default = (
        'b' =>  'default'
    );

    %args = (%args, %default);
    bless \%args, $class;
}
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The difference here is that the arguments hash, %args, is blessed as an object rather than singularly creating each element as was being performed. Also too, this approach lends itself well to argument validation, for example with the addition of a single validation loop:

use Carp;

%args = (%args, %default);
foreach (keys %args) {
    carp( 'Unknown argument to init subroutine - ', $_ ) unless exists
+ $default{$_};
}
bless \%args, $class;
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This approach allows for the keys in the passed %args hash to be validated against the %default hash so that only allowed elements are included in the object.