Perl interpolates variables in double quoted strings safely. Everything in the variable is considered to be a literal. So

my $a = q#${system('rm -rf /')}#;
print "The command in \$a is $a\n";
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will print out:

The command in $a is ${system('rm -rf /')}
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and not even attempt to remove my files. Likewise this

my $b = "The command in \$a is $a\n";
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sets $b to the string we printed out. Printing $b does exactly the same as the above. The only way this is going to come and cause us grief is if we eval $a - as you've suggested.

Perhaps the author is mistaken or you've misunderstood the reference, can you post the paragraph or two? I've checked the book errata and it's not mentioned anywhere there, but it's a pretty big mistake if you haven't misunderstood it.

Even authors make mistakes. :)