6.3. Test::More is( value, expected_value, name);

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

#!/usr/bin/perl
use strict;
use warnings;

use Test::More tests => 3;

is `./mycalc 1 + 1`,     2, '1+1';
is `./mycalc 2 + 2`,     4, '2+2';
is `./mycalc 2 + 2 + 2`, 6, '2+2+2';

Result

$ perl t7_calc.t 
1..3
ok 1 - 1+1
ok 2 - 2+2
not ok 3 - 2+2+2
#     Failed test (t7_calc.t at line 7)
#          got: '4'
#     expected: '6'
# Looks like you failed 1 tests of 3.

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

compares using eq