[instants-and-durations.t] Use .perl so as not to require great accur…

…acy from .Str.

S02-builtin_data_types/instants-and-durations.t

@@ -30,7 +30,7 @@ isa_ok eval('now +300'), Instant, 'now is a term, not a function';
     isa_ok $d + $t0, Instant, 'Instant + Duration ~~ Instant';
     isa_ok $d + $t0, Instant, 'Duration + Instant ~~ Instant';
     isa_ok $t0 - $d, Instant, 'Instant - Duration ~~ Instant';
-    is $t0 + ($t1 - $t0), $t1, 'Instant A + (Instant B - Instant A) == Instant A';
+    is ($t0 + ($t1 - $t0)).perl, $t1.perl, 'Instant A + (Instant B - Instant A) == Instant A';
     dies_ok { $d * $d }, 'Duration * Duration is illegal';
     dies_ok { $d ** 2 }, 'Duration ** Int is illegal';
     isa_ok 2 * $d, Duration, 'Int * Duration ~~ Duration';