What makes you think categories[some_tag] is not being submitted? I'd love to see the value returned by

$mech->response()->request()->as_string()
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Update: Works for me:

use strict;
use warnings;

use URI::file      qw( );
use WWW::Mechanize qw( );

my $mech = WWW::Mechanize->new();
$mech->get(URI::file->new_abs('file.html'));

$mech->form_number(1);
$mech->field("headline","MyHeadline");
$mech->field("thecontent","MyContent");
$mech->field("categories[some_tag]","Blah");
$mech->click("publish");

print $mech->response()->request()->as_string()
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<form method="POST">
<input type="text" name="headline" />
<input type="text" name="thecontent" />
<input type="text" name="categories[some_tag]" />
<input type="submit" name="publish" />
</form>
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POST file:///path/to/file.html
Accept-Encoding: gzip
Referer: file:///path/to/file.html
User-Agent: WWW-Mechanize/1.34
Content-Length: 70
Content-Type: application/x-www-form-urlencoded

headline=MyHeadline&thecontent=MyContent&categories%5Bsome_tag%5D=Blah
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>perl -wle"use LWP; print LWP->VERSION"
5.825

>perl -wle"use WWW::Mechanize; print WWW::Mechanize->VERSION"
1.34
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Thank you for your help.
Problem was much simpler:
within the form (60K) there was a hidden field; using this as field name instead worked...
Thanks again for your efforts.

Version is 1.34

use WWW::Mechanize;
   my $mech = WWW::Mechanize->new();
$mech->get("$mydomain/post.php");

$mech->form_number(2);
$mech->field("headline","MyHeadline"); 
$mech->field("thecontent","MyContent"); 
$mech->field("categories[some_tag]","Blah"); 
$mech->click("publish");
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HTML code is too large. Line in question is:

<input type="text" name="categories[some_tag]" class="categories form-
+input-tip" size="16" value="Add Word" />
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It is. Any solution?

Try using firefox w/Live HTTP Headers or Wireshark to see what gets sent by a live browser, then you can send the same thing using mechanize.