grep only goes through the actual elements of whatever is passed to it. No index is passed. You might want something like–

my @myArray = qw(alfa beta gamma);
grep { print "$myArray[$_] at $_\n" } 0 .. $#myArray;
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But that is *atrocious* style. grep is for filtering, not iterating. Use a for loop instead. Terse postfix version, you should generally prefer the expanded with a block–

print "$myArray[$_] at $_\n" for 0 .. $#myArray;
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