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multi sub plan(Cool:D :skip-all($reason)!)
multi sub plan($number_of_tests)

Specify the count of tests -- usually written at the beginning of a test file.

plan 15;   # expect to run 15 tests 

In subtests, plan is used to specify the count of tests within the subtest.

If a plan is used, it's not necessary to specify the end of testing with done-testing.

You can also provide a :skip-all named argument instead of a test count, to indicate that you want to skip all of the tests. Such a plan will call exit, unless used inside of a subtest.

plan :skip-all<These tests are only for Windows> unless $*DISTRO.is-win;
plan 1;
ok dir 'C:/'; # this won't get run on non-Windows 

If used in a subtest, it will instead return from that subtest's Callable. For that reason, to be able to use :skip-all inside a subtest, you must use a sub instead of a regular block:

plan 2;
subtest "Some Windows tests" => sub { # <-- note the `sub`; can't use bare block 
    plan :skip-all<We aren't on Windows> unless $*DISTRO.is-win;
    plan 1;
    ok dir 'C:/'; # this won't get run on non-Windows 
}
ok 42; # this will run everywhere and isn't affected by skip-all inside subtest 

Note that plan with :skip-all is to avoid performing any tests without marking the test run as failed (i.e. the plan is to not run anything and that's all good). Use skip-rest to skip all further tests, once the run has started (i.e. planned to run some tests, maybe even ran some, but now we're skipping all the rest of them). Use bail-out to fail the test run without running any further tests (i.e. things are so bad, there's no point in running anything else; we've failed).