Your code displays (on STDOUT as opposed to lwpcookies.txt):

Set-Cookie3: userpass="ikegami%25encodedpasswd%25salt"; path="/";
domain="www.perlmonks.org"; path_spec; discard; version=0
[download]

Note the "discard" flag. That means the cookie should only be used for as long as the browser isn't closed, and that it shouldn't be written to disk. LWP obeys that flag unless you add ignore_discard=>1 to the arguments passed to the HTTP::Cookies constructor. When I added ignore_discard=>1, the cookie showed up in lwpcookies.txt.

Who else thought it might have been Acme::Bleached? *wink*