Could the result you got back from the server you pasted in your first paragraph be the gzipped document you asked to download?
That server response looks much better that the "system unavailable" you got before trying Corion's suggestion.
Also use these setup in order to eliminate issues of rejection due to bad UAgent String and cookies turned off:
1) Use appropriate "user agent string" e.g.
my $mech = WWW::Mechanize->new( agent => 'Mozilla/5.0 (Windows NT 6.1;
+ WOW64; rv:47.0) Gecko/2010010
+1 Firefox/47.0' );
2) Use a cookie jar in order to save all the cookies for entire session leading to the file download,
my $mech = WWW::Mechanize->new( cookie_jar => {} );
