It would be much better to see the expected value and the actually received value. This usually helps in locating the problem.

examples/perl/t/32.t
use strict;
use warnings;

use lib 'lib';
use MyTools;

use Test::More tests => 3;

is(sum(1, 1),    2,     '1+1'   );
is(sum(2, 2),    4,     '2+2'   );
is(sum(2, 2, 2), 6,     '2+2+2' );

Output:

1..3
ok 1 - 1+1
ok 2 - 2+2
not ok 3 - 2+2+2
#   Failed test '2+2+2'
#   at t/32.t line 11.
#          got: '4'
#     expected: '6'
# Looks like you failed 1 test of 3.

See, in this case we can already guess that it cannot add 3 values.

compares using eq