sub foo_set {
    my ($self, $code)  = @_;
    my $code_ref = $self->can($code);
    if (defined($code_ref)) {
        $self->{'foo_code_ref'} = $code_ref;
    } else {
        die "Unable to handle '$code'";
    }
}

sub foo {
    my ($self, $arg) = _;
    my $code_ref = $self->{'foo_code_ref'};
    $self->code_ref($arg);
}