What on earth am I missing?

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]

In reply to Re: WWW::Mechanize trouble reading a text/plain file on a server by Anonymous Monk
in thread WWW::Mechanize trouble reading a text/plain file on a server by gargle

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