is($actual, $expected, $descriptionOfWhatYouTried);

   # to check effect of parameters passed to subroutine
   is(substr('abcdef', 0, 2), 'ab', q(tried subtr(..,0,2)});

   # to check what got captured by a regular expression
   is(eval { 'abcdef' =~ /^(\w+)/; $1 }, 'ab', q{tried /^\w+/});