What happens in case of
fox comes and the fox goes into forest
The code given above will put a the in front of every last fox irrespective of if a the was already there or not.

while(<DATA>) {
s/(?<!the )fox(?!.*fox)/the fox/ ;
print;
}
__DATA__
the fox comes and fox goes into forest
fox comes and fox goes into forest
the fox comes and the fox goes into forest
fox comes and the fox goes into forest
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Manav