You're asking for it to interpolate $1 in $r when you use double-quotes. Don't do this. $1 is undef when you do that, so of course, $r ends up being " world".

Here's a really crazy example of how to do what you want.

$test = "hello";
$s = "(hello)";
$r = '"$1 world"';
$test =~ s/$s/eval $r/e;
print $test;
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Note that this uses eval, which isn't everyone's thing.