I think your problem has to do with the following line: ($filePrefix_In, @entries_In, $yr_ref, $mth_ref, $dt_ref )  = @_.

What's going to happen is @entries_In is going to slurp up the remaining arguments, which means that $yr_ref, $mth_ref, and $dt_ref are going to be undef. You can solve this by reordering your arguments (i.e. ($filePrefix_In, $yr_ref, $mth_ref, $dt_ref, @entries_In)) or by passing the entries array by reference instead.