$str = "of course, japhy thought of this one";
$half = int(length($str)/2);

$str =~ s
  {( ?)([^ ]*)(?<=^.{$half})([^ ]*)( ?)}
  { length($2) < length($3) ? "\n$2$3$4" : "$1$2$3\n" }e;

print $str;

__END__
of course, japhy
thought of this one