This comment was in the "TO DO" section of the pod, meaning that the module author/maintainer was noting that he should do it, not you. Of course, you as the module "consumer" want to know that there may be a security problem in the module. The best thing to do in this case is to "Use the Source, Luke!":

The Source

There, you'll see the following little function, that might be of interest:

sub _cleanup_hdrs {
  my $hdrs = shift;
  my $h;
  foreach $h (values %$hdrs) {
    foreach (ref($h) ? @{$h} : $h) {
      s/\n\s*/ /g;
      s/\s+$//;
    }
  }
}
[download]

So, in my estimation, this is "DONE", and no longer "TO DO". Perhaps you should contact the maintainer and request that the pod be updated? That's your call...

Update: Oh yeah, forgot to mention that you should make sure that you've got the latest VERSION (1.65) of Mail::Mailer, so you can rest assured that you have got the fix.