You can return immediately if the string is less than or equal to the desired length. Then it depends on what you want to do to make it pretty. I have a couple of routines like this:

sub shorten_string {
    my ( $string, $length ) = @_;
    $length ||= 64;
    $length = 10 if $length < 10;
  return $string unless length($string) > $length;
    my $chunk = int($length/2)-3;
  return substr($string,0,$chunk+1) . ' ... ' . substr($string,-$chunk
+);
}

sub shorten_url {
    my ( $url, $length ) = @_;
    $length ||= 64;
  return $url unless $url and  length($url) > $length;
    ( $url ) = split /\?/, $url;
  return $url if length($url) < $length or $url =~ m!\w+://[^/]+/?$!;
    $url =~ s!(\w+://[^/]+)!!;
    my $domain = $1 ? "$1/ ... " : '... ';
    $length -= length $domain;
  return $domain if $length < 1;
    my @bits = split '/', $url;
    my @keepers = $url =~ m!/$! ? ('') : ();
    my $tail = 1;
    while ( my $bit = pop @bits ) {
        next unless $bit;
        $length -= (length($bit) + 1);
        unshift @keepers, $bit if $tail or $length > 0;
        $tail = 0;
      last if $length < 1;
    }
  return @keepers ? ( "$domain/".join'/',@keepers ) : $domain;
}
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