##

    package Model::Validate::Foo;

    sub on_insert {
        my %value_of = @_;

        if ( !$value_of{a} ) {
            die "a is required";
        }

        if ( $value_of{b} !~ /\d+\.\d+\.\d+/ ) {
            die "invalid b";
        }

        if ( !$value_of{c} ) {
            die "c is required if a" if $value_of{a};
        }
    }

##

##

    sub avg_a {
        # called on insert, update of table foo
        # check if average(a) > 100
    }