in reply to
how to update a field of a record in a large text file
assuming you are familiar with the use of regular expressions (see 'perldoc perlre' if not), what you want is to perform a substitution using regular expression on this record.
to do it on a text file - I recommend tie'ing the file to an array and perform the substitution on the array.
take a look at Tie::File.
tie @array, 'Tie::File', 'test';