redo entire loop

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi, if I want to modify a record in a file and I don't know whether the record I'm looking for already exists can I do that in one pass without having to write out the whole file. e.g. This is the pseudo code

open file
While (<file>)
{
if value exists then 
modify and print record to output file
else
print record to output file
}
rename output file back to original filename
[download]

If I do this and the value I'm searching for doesn't exist then I've re-written the entire contents of the original file needlessly. Hence is there anyway to redo the same loop so the first time round I see if the value exists and if it doesn't I don't bother creating an output file ? So a redo for the whole loop rather than the current iteration of the loop ?

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