If you need to look at the source code of a module, the author failed in writing their documentation. If Perl was C, it would be harder to not document your code since the parameters and return value can't be hidden. In Perl both are hidden. In writing docs, I basically address the following questions.
What are the parameters?
how many?
variable parameters?
what parameter validation is performed?
how is error condition indicated? die/exceptions? warn? $!? return value?
What is the return value? list or scalar?
what are the possible errors (if possible, sometimes they come from something your module calls)?
common ways to leak resources
common mistakes
does undef as a parameter have special meaning?
what the function DOES NOT do
alternatives in same module, other modules or perl language
if XS and wrapping a C lib, situations that create unavoidable crashes or bugs in the C lib you as the module author hit