I'm pretty sure that's because your first call of sub_c happens before it is defined, hence before its prototype is known.

The best solution is not not use prototypes, but instead pass a hash reference to sub_c, e.g. sub_c(\%hash1b);

A quick fix would be to place a "sub sub_c (\%);" before sub_b. Or, you can move sub_c before sub_b, but that won't help if the call tree is more complex than in your example.