Well, I think you missed the essence of my question. "shell escapes" are straight from the perldata manpage:

As in some shells, you can enclose the variable name in braces to disambiguate it from following alphanumerics (and underscores). You must also do this when interpolating a variable into a string to separate the variable name from a following double‐colon or an apostrophe, since these would be otherwise treated as a package separator.

$a[$b]  is the 6th element of the @a array

${a}[$b] is supposed to be the same as

join '', $a, '[', $b, ']';
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but if you declare the variable $a to be lexically scoped with my then perl seems to want to force them the construct to be considered as an array, which is wrong.