This happens with flake jobsets for obvious reasons (namely, that nixexprinput and nixexprpath may be undefined for a flake jobset).

12:38:59 hydra-evaluator.1 | Use of uninitialized value $args[0] in join or string at /home/vin/workspace/vcs/hydra/src/script/hydra-eval-jobset line 648. 12:38:59 hydra-evaluator.1 | Use of uninitialized value $args[1] in join or string at /home/vin/workspace/vcs/hydra/src/script/hydra-eval-jobset line 648.