In both cases the compiler might report an error where the mistake was inserted, rather than where it actually is.

Well, after the file is filtered and text inserted via inline directive expansion, the erroneous code is duplicated and defacto is actually also at the new location. That's how inlining works. Anybody using a source filtering module should know what they are doing. The confusing line numbers issue can be mitigated by running -MO::Deparse,-l on the faulty file, and the line numbers reported by the compiler will match those of the output.

This is precisely why I included the debug switch into the module configuration, which causes the output of the code after mangling. But yes, there are much more CAVEATS than those currently mentioned in that section.

Thanks for your input!