In general, your tests shouldn't produce warnings. This modules causes any
warnings to be captured and stored. It automatically adds an extra test that
will run when your script ends to check that there were no warnings. If there
were any warings, the test will give a "not ok" and diagnostics of where, when
and what the warning was, including a stack trace of what was going on when the
it occurred.

If some of your tests are supposed to produce warnings then you should be
capturing and checking them with Test::Warn, that way Test::NoWarnings will not
see them and so not complain.

The test is run by an END block in Test::NoWarnings. It will not be run when
any forked children exit.