In perlsyn (under Switch statements) there's a sub-section called Smart matching in detail. Here you'll find a table of all the smart match combinations. They're all boolean.

Index doesn't even make sense for most smart-match inputs, so it doesn't make sense for smart-matching in general.

This is probably what you want:

my @indexes = map { EXPR ~~ $a[$_] } 0..$#a;
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hopefully we're not arguing semantics...but i think being able to get the index (array 'position') based on the result of a positive smartmatch is very useful. knowing that ( "foo" ~~ @big_array ) is true is helpful (and we rejoice in how fast it is). in some cases knowing that "foo" is at index 346456 may be really really helpful. i realize that "foo" could appear repeatedly and in that case the map example is a good solution. it would be nice if the smartmatch function could be a bit more "telling".

i think being able to get the index (array 'position') based on the result of a positive smartmatch is very useful.

Indeed, and I showed you how to do it.

it would be nice if the smartmatch function could be a bit more "telling".

Smart match has nothing to do with arrays. It can't return indexes when most match rules don't even involve indexes.