You could describe the rules in XML:

    <table name="foo">
        <field name="a" required="1"/>
        <field name="b" validate="\d+\.\d+\.\d+"/>
        <field name="c" required="if a"/>
    </table>
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These could be easily translated to perl:

    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};
        }
    }
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Or, perhaps you could have constraints declared like this:

    <table name="foo">
        ...
        <constraint name="avg_a" value="average(a) > 100"/>
    </table>
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Which would generate a sub like:

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