Just negate the test. 'ok' is looking for the test to succeed. If you are testing a sub and are looking for it to fail then re-write the test such that the failure of the sub is a succesful test. 
# this one should succeed when the sub fails
ok(!my_sub('bad parameter count'), "Bad param test");

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In reply to Re: Test::More : I'd need a "not_ok()" function... by inman
in thread Test::More : I'd need a "not_ok()" function... by wazoox

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