Try

$string =~ s/^(.{0,$maxlength})\b.*$/$1.../s;
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that should work as expected (at least as far as the presented code goes...).

Edit: explanation: you can ommit the upper bound, but not the lower bound, as you did. From perlre:

     {n}?   Match exactly n times
     {n,}?  Match at least n times
     {n,m}? Match at least n but not more than m times
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and you have to match the rest of the input (.*$) for it to be replaced too - otherwise you would simply insert the ellipsis (...) without truncating the input.