When asking for help it's kind of important to provide the exact text of the error message (let alone a code sample which consistently reproduces the error message, but I digress . . .), otherwise it's going to be a crapshoot with people reduced to guessing as to what the actual error message really was.

Looking in perldiag the closest I can find is the message when you've used a subroutine name without parens around its argument that hasn't yet been declared (either in the source in question or other code require'd or use'd lexically prior to the call in question). The quickest fix is to wrap the arguments in parens and be done with it; baring that use one of the perldiag suggestions and predeclare it (sub foo;) or move the actual declaration lexically before the invocation.

As for prototypes, unless you're trying to mimic a perl builtin and/or really know what you're doing you don't want to mess with them.