Perhaps they only have core modules on their setup for testing.

Well, yes. How else could they test that all the dependencies have been correctly listed? The system is working as intended :-)

It seems, quite a few testers don't have Test::Warn installed...

You should be able to detect that and skip the test(s).

Very crudely, something like this should work (completely untested):

#...

use Test::More tests => 42;
eval 'use Test::Warn;'; # just ignore if we can't load Test::Warn

#...

SKIP: {
    unless ($INC{'Test/Warn.pm'}) { # test if Test::Warn was loaded
        skip 'Test::Warn is not available', 1;
    }
    warning_like { warn "foo" } qr/foo/; # you may need to put this li
+ne in a string eval because warning_like is exported by Test::Warn
}

# ...
[download]

Or maybe put all warnings tests in a separate script and skip the entire script if Test::Warn is not available (again untested):

# ...

use Test::More;
BEGIN {
    if (eval 'use Test::Warn; 1') {
        Test::More->import(tests => 42);
    } else {
        Test::More->import(skip_all => 'Test::Warn is not available');
        # this line won't be reached, see Test::More
    }
}

warning_like { warn "foo" } qr/foo/; 

# ...
[download]

Alexander