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#!/usr/bin/perl

...
    -name => "foo",
    -default => "bar",
), "\n";

> ./hidden.pl # the code above
<input type="hidden" name="foo" value="bar" />
> ./hidden.pl foo=123
<input type="hidden" name="foo" value="123" />

Note, that just like all the other form elements, the value of a
hidden field is "sticky".  If you want to replace a hidden field
...
have to do it manually:

     $query->param(hidden_name,new,values,here);