"I would like to ask what is "caution" here?"

package Person {
  use Moose;
  has name => (is => 'ro', isa => 'Str');
  sub introduce_yourself {
    my $self = shift;
    printf("My name is %s\n", $self->name);
  }
}

package SecretAgent {
  use Moose;
  extends 'Person';
  # secret agents have many aliases
  has '+name' => (isa => 'ArrayRef');
}

my $bond = SecretAgent->new(
  name => ['James Bond', 'Burt Saxby', 'David Somerset'],
);
$bond->introduce_yourself;
[download]

You see the problem?

Making a type constraint tighter (e.g. if the parent class wants a Num, and the child class restricts it to an Int) should usually be just fine. Making it looser requires more caution. The author of the SecretAgent class needs to check which methods of Person assume that name is a string, and override them all.

package SecretAgent {
  use Moose;
  extends 'Person';
  # secret agents have many aliases
  has '+name' => (isa => 'ArrayRef');
  sub introduce_yourself {
    my $self    = shift;
    my @names   = @{ $self->name };
    my $name    = $names[ rand @names ];
    my $surname = (split / /, $name)[-1];
    printf("The name's %s, %s\n", $surname, $name);
  }  
}
[download]