I don't know what can be wrong...

What about following code?

...
use utf8;
my $max_text_len = 40;
...

sub split_text {
  my $intext = shift || return '';
  my @result = ();
  my $index  = 0;

  foreach my $word (split(' ', $intext)) {
    next if !length($word);

    if (length("$result[$index] $word") > $max_text_len) {
      $index++;
      $result[$index] = $word;
    } else {
      $result[$index] .= ($result[$index] ? ' ' : '').$word;
    }
  }
  return join("\n", @result);
}
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