I'd try to simplify the "unless" clause, just to make it easier to comprehend:

...
    unless ( $resfile eq "_RES_NODEFAULTS" ) {
       # do the normal (default) resource inits...
    }
    if ( $resfile and $resfile ne "_RES_NODEFAULTS" ) {
       # read resources from $resfile...
    }
...
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As for the documentation, the hierarchy (1.1 vs. 1.2 vs. 2) doesn't really help. There are just three different possible invocations for "new()", involving the single argument that it can take:

• the literal string "_RES_NODEFAULTS" -- returns a completely empty object, with no resource values set
• no argument at all (or empty string) -- assigns default values to a core set of resources
• the name of a resource file -- can assign non-default values to core resources, and/or define additional resources

I haven't looked at the rest of the module, so I'm guessing that those descriptions are appropriate. In any case, I hope the docs will clarify what the "core" resources are, how the default values make sense, what a resource file should look like, and what constraints, if any, apply to defining additional resources.