Module::install::extratests adds one module::install command:

extra_tests;
this declares that the test files found in the directory ./xt should be run only in certain instances:

./xt/author - run when tests are being run in an author's working copy
./xt/smoke - run when the dist is being smoked (automated_testing=1)
./xt/release - run during "make disttest"
these directories are recurisvely scanned for *.t files. if any directories or files exist in ./xt that are not recognized, the makefile.pl will die.