Something

CREATE OR REPLACE FUNCTION marc_is_valid (TEXT) RETURNS BOOLEAN AS $$
        use MARC::Record;
        use MARC::File::XML (BinaryEncoding => 'UTF-8');
        my $record;
        eval {
                $record = MARC::Record->new_from_xml($_[0])
        };
        return if $@;
        return 1;
$$ LANGUAGE plperlu;