5.4. Method modifiers

before()

Run a coderef before calling the method being modified. Things to note:

1. The coderef receives the same params as the original method.
2. The return value of the coderef is ignored.
3. before is good for adding extra validation

Example:

package Brontosaurus;
use Moo;
extends 'Dinosaur';

before eat => sub {
    my ($self, $food) = @_;

    die "bad params" if $food eq 'meat';
};

after()

Run a coderef after calling the method being modified. Things to note:

1. The coderef receives same params as the original method.
2. The return value of the coderef is ignored.
3. after is good for logging/debugging.

Example:

package Brontosaurus;
use Moo;
extends 'Dinosaur';

after eat => sub {
    my ($self, $food) = @_;

    $self->log->warning("Brontosaurus does not like to eat $food")
        if $food eq 'meat';
};

around()

Run a coderef instead of the method being modified. Use around to:

1. Pass the original method modified parameters.
2. Change the return value of the original method.
3. Avoid calling the original method conditionally.

Example:

package Brontosaurus;
extends 'Dinosaur';

around eat => sub {
    my ($orig, $self, $food) = @_;
    uc $orig->($self, $food);
};

More details

See https://metacpan.org/pod/Class::Method::Modifiers