Which to me suggests that the shell does some processing with double-quoted strings before it reaches perl
That depends on the shell. Typically, Unix shells with do interpolation (including `` expanding) inside double quoted strings, and no interpolation inside single quoted string.

It's my understanding that you only have double quoted strings on most Windows shells (not counting shells that started life on Unix, then got ported). No idea whether they do interpolation or not.

In reply to Re^2: Choosing regex delimiters by JavaFan
in thread Choosing regex delimiters by xyzzy

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