Similar problem, different context. In perl, double quotes causes interpolation, so in

$r = "$1 world";
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$1 is substituted with whatever was matched in a previous regex. To prevent interpolation, use single quotes.

That doesn't solve the whole problem, however. Now we need to substitute the strings, including metacharacters, into s///. To interpolate, then substitute, I'll use an eval:

$s = '(hello)';
$r = '$1 world';
$_ = 'hello';
eval "s/$s/$r/";
print $_;
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A good general rule to follow is to use single quotes if you want the string as is, and use double quotes if you want interpolation.

-Mark