examples/perl/t/isnt_undef.t
use strict;
use warnings;

use Test::More tests => 2;

my $x;
isnt($x, undef);

$x = 1;
isnt($x, undef);

Output:

1..2
not ok 1
#   Failed test at t/isnt_undef.t line 7.
#          got: undef
#     expected: anything else
ok 2
# Looks like you failed 1 test of 2.