duff , sorry if my question was not clear , ok lets put it this way what if I want to just do : my input fils has only the following

    export O_PREFIX=OLD
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I am doing the follwoing to change the value from OLD to NEW:

my $newValue='NEW';
s/O_PREFIX=(\w+)/O_PREFIX=$newValue/ if /O_PREFIX=/;
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my code is working and I am reading from the correct file , however my output looks like this

O_PREFIX=NEWOLD
instead of 
O_PREFIX=NEW
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I hope this make it clear , if not then sorry :(