DBIx::ProcedureCall needs to know if you are about to call a function or a procedure (because the SQL is different). You have to make sure you call the wrapper subroutines in the right context:

You have to call procedures in void context.

You have to call functions in non-void context.

If you do not want to rely on this mechanism, you can declare the correct type using the attributes :procedure and :function:

use DBIx::ProcedureCall qw[
  sysdate:function
  dbms_random.initialize:procedure
];
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If you use these attributes, the calling context will be ignored and the call will be dispatched according to your declaration.