Thanks for replying!
Your snippet was useful. I did add the line and the program doesn't terminate but it still don't work. Would you happen to know any quirks of WWW::Mechanize module, like whether its subroutines cannot be passed with certain kind of string??
This is weird, when I add  die unless $title eq 'whatever'; the processing doesn't terminate and the program doesn't work, but when I make explicit assignment after your line as:

die unless $title eq 'whatever';
$title = 'whatever';
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It works. So... o_0
I even added following lines:

if ((defined($title))&&($title ne ""))
        {
                print "\nTitle filled with ... $title";
                $mech->field(title => $title);        
        }
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The output says: Title filled with ... whatever
and then.. LWP::UserAgent::request: Simple response: Internal Server Error