What's in $i? It should work fine (and does for me) when $i eq "\n". 
$rhs .= "hello";

$i = "\n";

        if ($i =~ m/[ -~]/)
        {
            $rhs .= $i;
        }
        else
        {
            $rhs .= ".";
        }

$rhs .= "world";

print("$rhs\n");   # Prints: hello.world

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You regexp reads as "contains a valid char". Maybe you need one of the following (equivalent to each other) regexps.
$i !~ m/[^ -~]/     # Doesn't contain invalid chars
$i =~ m/^[ -~]*\z/  # Only contains valid chars

In reply to Re: regex incorrectly matching newline by ikegami
in thread regex incorrectly matching newline by bfdi533

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