$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
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This determines the midpoint of your string, and then looks for a space (if there is one) followed by non-spaces, stopping at the middle of the string, and then looks for non-spaces followed by a space (if there is one). Then, based on the lengths of the non-space sections, it determines where to insert the newline. Centering this content, if wanted, is then child's play.