Hi - thanks for checking my code. This code worked for you? I copied exactly what you posted (well, I changed $mech->submit_form to $mech_obj->submit_form) and I still got the same error:

Unexpected field value http://search.cpan.org at (eval 5) line 1
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It sounds like there is something else going on at my end. I'll have to keep digging. Thanks again for checking this out for me.

Update: I added use Carp 'verbose' to the test script and got this:

Unexpected field value http://search.cpan.org at C:/ActivePerl/site/li
+b/HTTP/Headers.pm line 256

HTTP::Headers::_header('HTTP::Headers=HASH(0x2249820)', 'Referer', 'ht
+tp://search.cpan.org')  called at C:/ActivePerl/site/lib/HTTP/Headers
+.pm line 150

(etc)
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The error is coming from the _header subroutine in HTTP::Headers. The code in question is this:

if (!ref($val)) {
    push(@new, $val);
} elsif (ref($val) eq 'ARRAY') {
    push(@new, @$val);
} else {
    Carp::croak("Unexpected field value $val");
}
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I added a Data::Dumper print statement to determine what $val was, and I got the following:

$VAR1 = bless( do{\(my $o = 'http://search.cpan.org')}, 'URI::http' );
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which matches the "action" value in the $mech_obj above. The _header routine seems to want scalars (or a ref to an array), but it looks like it's getting a hash ref (or is it the whole object?) from the WWW::Mechanize object instead.

I was not sure how to proceed, so emailed the authors of those modules. Additional suggestions are welcome. (Incidently, now that I know more about the error, I found a previous report of it in HTTP::Message.)