I'm not sure I like any of the solutions yet offered (can you tell?) so I whipped this up: 
sub shorten {
  local $_ = shift;
  my $len  = shift;
  my $pos;
  
  return $_ if $len >= length;

  $len -= 3;
  $pos = pos while /\w(?=\s)/g && $len >= pos;
  $pos ||= $len;
  substr($_, 0, $pos)."...";
}

print shorten("misunderestimate", 15), "\n";
print shorten("all work and no play makes MeowChow a dull boy", 15), "
+\n";
print shorten("imapirateiamiam", 15), "\n";

[download]
If you don't care about matching multiple spaces or about matching tabs, I would rewrite the second and third-to-last lines in the sub as follows (which would essentially make it bbfu's solution): 
  $pos = rindex $_, " ", $len;
  $pos = $len if $pos < 0;

[download]
This would make a good Golf contest by the way (hint, hint)... 
   MeowChow                                   
               s aamecha.s a..a\u$&owag.print

In reply to Re: Smart Substrings by MeowChow
in thread Smart Substrings by cei

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.