Type::Tie

   use strict;
   use warnings;
   
   {
      package Local::Testing;
      use Moose;
      has numbers => ( is => "ro", isa => "ArrayRef[Num]" );
   }
   
   # Nice list of numbers.
   my @N = ( 1, 2, 3, 3.14159 );
   
   # Create an object with a reference to that list.
   my $object = Local::Testing->new(numbers => \@N);
   
   # Everything OK so far...
   
   # Now watch this!
   push @N, "Monkey!";
   print $object->dump;
   
   # Houston, we have a problem!

   use Type::Tie;
   use Types::Standard qw( Num );
   
   ttie my @N, Num, ( 1, 2, 3, 3.14159 );

   use Type::Tie;
   use Types::Standard qw( Int Num );
   
   ttie my $count, Int->plus_coercions(Num, 'int $_'), 0;
   
   $count++;            # ok
   $count = 2;          # ok
   $count = 3.14159;    # ok, coerced to 3
   $count = "Monkey!";  # dies