After ruling out the obvious, I used Charles proxy to see the decrypted request each method was making and it looks like AnyEvent::HTTP is probably mangling the request.

This is what the working request looks like: 

POST /post HTTP/1.1
TE: deflate,gzip;q=0.3
Connection: TE, close
Accept-Encoding: gzip
Host: h.wrttn.me
User-Agent: WWW-Mechanize/1.72
Content-Type: application/x-www-form-urlencoded
Rest-Key: key
Rest-Sign: eZsAHbbDBmD3/vHe7zg8IDdPJlV8WHBgHiAdd+ZDb8cD9TPHza4liDAYiLM
+52nHLaGF2Jb9hlpoF 72SfxUg1Uw==
Content-Length: 34

pass=&nonce=1351623186546267&name=

[download]
And this is the mangled request from AnyEvent::HTTP: 
POST /post HTTP/1.1
User-agent: Mozilla/5.0 (compatible; U; AnyEvent-HTTP/2.14; +http://so
+ftware.schmorp.de/pkg/AnyEvent)
Connection: close Te
Rest-sign: GvzSzGg5WHPpL4O2bw48Q8lX6O5prasaiysH98FVS9oWYNfcs7pdUtWNpm7
+xGYG8zT3e0/BcPgHP LZWzMTdubg==

[download]
I see the same behavior when changing from https to http.

In reply to Re: Calling LWP/Mech from AnyEvent by Anonymous Monk
in thread Calling LWP/Mech from AnyEvent by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.