my $str = '1234 6789 1234 6789 1234 6789 ';
my $printstr = substr($str, 0, 20+1);
$printstr =~ s/\W+\w*$//;
print $printstr;
[download]

Unless I'm missing something important, this should cut the line at the right spot. Note that it's 20+1 because the length has to be max length + 1 so it won't cut a word that fits entirely into the string.

EDIT: You may have to change the regex if words can include characters other than \w, such as .