First pad out or trim the replacement text to 10 characters with pack,

$repl = pack 'A10', $repl;
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then do the replacement with substr,

substr $str, 10, 10, $repl;
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or,

substr($str, 10, 10) = $repl;
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Thare are dozens of other ways to do it, and you don't need to pad or trim if that's not what you want.