The string/number heuristic is responsible for some of the oddest behaviour of smart match. Let's take for example the use of $x ~~ @array as an "in" operator.

sub as_bool { $_[0] ? "true" : "false" }

my @greetings = qw( Hi Bye );
say as_bool("Hi" ~~ @greetings);     # true
say as_bool("Hiya" ~~ @greetings);   # false
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OK, everything is working as expected so far. Now let's do something a little more complex...

push @greetings, 0, 1;
say as_bool(0 ~~ @greetings);       # true
say as_bool(1 ~~ @greetings);       # true
say as_bool(2 ~~ @greetings);       # false
say as_bool("Hiya" ~~ @greetings);  # true ?!?!
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Not having memorized the smartmatch table, that looked pretty strange to me at first, but looking at the table, at least it's well-defined... and when I tried it I got a "Argument "Hiya" isn't numeric in smart match" warning, which (although it's not much) is better than it silently matching. But the problem of when to match with == vs. eq remains...