If you are using CGI.pm, it has the feature of "sticky fields", that is, fields that retain their value on output. This is in most cases a very welcome addition as the user dosen't have to type in information again just because the CGI script didn't accept the data as a whole.

To switch off this very convenient behaviour, you can use :

use CGI qw(:standard -nosticky);
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or if you want to replace a single form field value with another value from your script, the CGI.pm documentation tells you the following way:

   $query->param('field_name','new value');
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