I recommend you use MIME::Base64 to base64-encode the HTML, and include the "content-encoding: base64" header.

Otherwise, you will find that the HTML gets some extra newline characters where you don't want them (i.e. - in a hyperlink or image source attribute).

So, just this line of code
print $sock "Content-type: text/plain\n\n";
wouldn't make it work.