Just a note about style and readability (though I agree that File::Temp sounds like a good idea) ...

Since unless is discouraged in cases where it's potentially confusing (instead you can use while (not ...)), and since you already have a negative in the command (the "!" in (! -f "$md")), I would strongly prefer writing it as:

    do {
        $tmpname = int(rand($limit));
        $md      = "$TMPDIR/runitout_$tmpname";
    } while (-f $md);
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Note, too, that you don't need quotation marks around $md.