Maybe I'm missing something?

Only that I was just providing a starting point to illustrate a method. I figured you would tweak it to meet your needs. But in fairness, I should have provided an answer that produced exactly what you were looking for. I've modified my answer to do what I think your question was asking, and will paste it below...

The problem with my original (I think) was that it broke your original string into substrings of 'n' characters. What you were looking for was simply truncation, with trailing '...' elipses. Here ya go...

use strict;
use warnings;

my $len = 40;
print "0123456789012345678901234567890123456789\n";
while ( my $string = <DATA> ) {
    chomp $string;
    $string =~ 
        s/((?:.{0,$len}(?=\s|$))|(?:\S{$len}))(?:.+$)?/$1.../s;
    print "$string\n";
}

__DATA__
Now is the time for all good men to come to the aid of their country.
Nowisthetimeforallgoodmentocometotheaidoftheircountry.
[download]

Hope this helps.