Method modifiers (before/after)

• add some extra checking before or after a method runs

package Odd::Class;
use Moose;

has foo => ( is => 'rw' );

before foo => sub {
    confess 'You can only read or write foo when the time is even'
      unless time % 2 == 0;
};

after qw/foo expensive_calculation do_stuff/ => sub {
   my $self = shift;
   Carp::cluck 'CPU limit exceeded' if $self->cpu->limit->is_exceeded;
   exit(1); # you die
}

• You can't modify args or return value
• Only side-effects