$a = qq|${system("echo 'hello from system'")}\n|;
$a = "${system(\"echo 'hello from system'\")}\n";
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The book seems to imply that this behavior emulates what would happen if $a had come from user input. Fortunately that is not the case. If $a had come from STDIN, none of the above caveats would apply. Try it:

#!/usr/bin/perl -wT
use strict;

my $a = <STDIN>;
chomp($a);
print "$a";
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-Blake

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.