Thanks K! Yours was the only information I could find on the data::table join operator anywhere on the internet! This is what I ended up using: $results_by_species_tabref = $results_by_species_tabref -> \ join($sp_domcount_tabref, 3, "HMM", "HMM" ); I was not enough of a perlista to recognize that "ColumnName" would give the references the operator required. The authors did not provide an example of this type this on CPAN.