Proof :D Maybe dump the headers of the response where you use :content_file, you might see a difference.

There really are no red flags in what you've shown, my conclusion, some servers are just broken.

It pays to always upgrade WWW:Mechanize/LWP to the latest

$ pmvers WWW::Mechanize LWP HTTP::Headers HTTP::Request URI HTTP::Resp
+onse
WWW::Mechanize: 1.68
LWP: 6.02
HTTP::Headers: 6.00
HTTP::Request: 6.00
URI: 1.58
HTTP::Response: 6.01
[download]