jack3:/usr/genoais/aiq/aiq/bin$ od -c auto_20090318_0610.zip | head -n
+ 4
0000000   P   K 003 004 024  \0  \0  \0  \b  \0   a   I   s   :   S   
+F
0000020 215 200   C   & 005  \0 345 233   *  \0 026  \0  \0  \0   a   
+u
0000040   t   o   _   2   0   0   9   0   3   1   8   _   0   6   1   
+0
0000060   .   c   s   v 344 275   [   s 333 310 226   . 370   > 021 36
+3
jack3:/usr/genoais/aiq/aiq/bin$ od -c report.zip | head -n 4
0000000   P   K 003 004 024  \0  \0  \0  \b  \0   a   I   s   :   S   
+F
0000020 357 277 275 357 277 275   C   & 005  \0 357 277 275 357 277 27
+5
0000040   *  \0 026  \0  \0  \0   a   u   t   o   _   2   0   0   9   
+0
0000060   3   1   8   _   0   6   1   0   .   c   s   v 357 277 275 35
+7
[download]

"\215" (and basically every other byte ≥128) are being replaced with "\357\277\275". "\357\277\275" is the UTF-8 encoding of \x{FFFD}, the replacement character used to represent bad data.

$ perl -MEncode -e'printf "%04X\n", ord decode "UTF-8", "\357\277\275"
+'
FFFD
[download]

It sounds like something tried to decode the zip file.

$ perl -MEncode -we'print decode "UTF-8", "\215"' | od -c
Wide character in print at -e line 1.
0000000 357 277 275
0000003
[download]

While WWW::Mechanize's content calls decoded_content (defined in HTTP::Message), decoded_content shouldn't attempt to decode a zip file (only files with MIME type text/*).

Is the web server incorrectly saying the .zip is a UTF-8 text file? Could you provide the output of the following:

print $mech->response()->headers()->as_string();
[download]

Delete "Set-Cookie:" headers and other authentication data before posting.

Does this help? This is a Dumper($mech):

                                                          'content-typ
+e' => 'text/html; charset=utf-8',
                                                         'server' => '
+Microsoft-IIS/6.0',
                                                         'content-styl
+e-type' => 'text/css',
                                                         'x-are' => 'y
+ou digging my headers?',
                                                          'x-powered-b
+y' => [
                                                                      
+        'http://www.bandwidth.com',
                                                                      
+        'ASP.NET'
                                                                      
+      ],
                                                          'content-dis
+position' => 'attachment;filename=auto_20090318_0610.zip',
                                                          'client-resp
+onse-num' => 1,
                                                          'content-len
+gth' => '337991',
                                                          'x-aspnet-ve
+rsion' => '2.0.50727',
[download]

I notice the content-type and content length.

Jack