Neat idea. A few FYIs:

Requests to perlmonks can be made faster by asking for very small nodes. It's not necessary to use node 109 for login or homenode for /msg (or lastnode_id at all). If a node isn't supplied The Monastery Gates comes back (ouch). The smallest nodes at perlmonks are some empty XML nodes but I usually use the tiny XML xp nodelet at node_id=16046 since it'll probably be there in the future.

If you have a saved cookie file login can be skipped, but either needs to use ignore_discard on the cookies:

$co=new HTTP::Cookies(file=>'./cookie',autosave=>1,ignore_discard=>1);
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or set an expires param in the login query:

$url->query_form(
  op => "login",
  node_id => "16046",
  expires=> "+10y"
  user => $pm_user,
  passwd => $pm_password
);
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Also, the size limit on /msg is 255 bytes, so $message needs to be counted and truncated before the print to reflect overflow.