Opps must have foobared my test. You are correct

$a = qq|${system("echo 'hello from system'")}\n|;
$a = "${system(\"echo 'hello from system'\")}\n";
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do the same thing. Some how I had your example simply printing the string... hmmm

Anyway I read the author to mean to not do this:

$a = <stdin>;
print "Some string with ${$a}";
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But as you stated I'm finding that doesn't work, which is very strange to me. It seems as though it should, and there could be really cool uses for it.

That said, to get somewhat back on the orginal topic using taint mode is still a good idea, especially in CGI scripts. The trick is to learn what should be untainted and what doesn't have to be.