Newly added to the Perl Language Support in RSTALanguageSupport is the ability to modify the value of the PERL5LIB environment variable used when syntax-checking source code. This feature has already been integrated into the RText Subversion trunk:
Missing a Perl library
Adding it via PERL5LIB
Problem solved!
As the source code in the screenshots say, you can also solve this (a more proper solution in many cases, as well) with “use lib”, but I’m trying to give as much freedom with the language support library as possible.