Perl used to have ".", the current directory in @INC, the list of directories it uses to load modules from. due to security concerns in Perl 5.26 it was removed. That means code that relied on "." will break when you upgrade to Perl 5.26.

The Perl delta explains how to deal with the removal safely and not so safely.

As of 29 November 2017 when you use Travis-CI, the leading Continuous Integration system of the Open Source world, you will see that the PERL_USE_UNSAFE_INC environment variable was set to 1 that tells Perl to add "." back to @INC restoring the potential security issue. This is however, as I learned after reporting the issue, is not done by Travis itself. It was done by Test::Harness that is used in the "make test" phase.

I think this is a bad idea.